Creamy Brussels Sprouts and Mushroom Casserole with Crispy French Fried Onions: Your New Favorite Thanksgiving Side Dish

Are you ready to elevate your holiday meal? This extraordinary Brussels Sprouts and Mushroom Casserole, crowned with a irresistible layer of crispy French fried onions, is not just a side dish; it’s a culinary revelation. It offers a fresh, vibrant, and incredibly savory twist on the traditional Thanksgiving spread that will have everyone asking for the recipe. Forget everything you thought you knew about green bean casseroles – this from-scratch creation is here to steal the show!

When the holidays approach, the dilemma of choosing the perfect Thanksgiving side dishes can be daunting. Do you stick to the beloved classics that grace your table year after year, or do you venture out and explore new flavors and textures? While traditional sides hold a special place in our hearts, there’s always room for innovation that truly enhances the festive feast.

If you’re looking to introduce an exciting, yet comforting, new favorite to your holiday menu, then this Brussels sprouts and mushroom casserole is an absolute must-try. It’s a delightful and much-loved variation on the classic green bean casserole, offering a richer, more complex flavor profile. Made entirely from scratch, it features tender Brussels sprouts and earthy mushrooms bathed in a luxurious, creamy sauce, all finished with that essential crispy French onion topping. It’s the epitome of cozy, savory, and utterly delicious comfort food, promising to be a highlight on any holiday table.

Why This Brussels Sprouts and Mushroom Casserole Will Be Your New Holiday Staple

This isn’t just another side dish; it’s a thoughtfully crafted experience designed to impress without overwhelming your holiday cooking schedule. Here’s why this creamy Brussels sprouts and mushroom casserole stands out:

• Unforgettable Flavor Profile: The combination of slightly caramelized Brussels sprouts, umami-rich mushrooms, and a velvety homemade cream sauce creates a depth of flavor that’s both sophisticated and comforting.
• A Modern Twist on Tradition: While it pays homage to the beloved crispy onion casserole, using Brussels sprouts and fresh mushrooms elevates it to a gourmet level, offering a healthier yet equally indulgent alternative.
• Homemade Goodness: Everything, from the sautéed vegetables to the creamy base, is made from scratch. This allows for superior taste and control over ingredients, ensuring a fresher and more vibrant dish than those relying on canned soups.
• Irresistible Texture: You get the best of all worlds – tender-crisp Brussels sprouts, soft sautéed mushrooms, a smooth, luscious sauce, and a perfectly golden, crunchy topping of French fried onions and panko breadcrumbs.
• Perfect for Planning Ahead: Holiday meals are busy! This casserole can be prepared in advance, freeing up valuable time on Thanksgiving Day, making it a stress-free addition to your menu.

Essential Ingredients for a Flavorful Brussels Sprouts and Mushroom Casserole

Creating this stunning Brussels sprouts and mushroom casserole starts with selecting fresh, high-quality ingredients. While the full recipe card below lists exact measurements, let’s delve into the key components that make this dish so special:

• Brussels Sprouts: Fresh Brussels sprouts are essential here. Look for firm, bright green sprouts without yellowing or loose outer leaves. Halving them allows for better caramelization and ensures they cook evenly.
• Cremini or Baby Bella Mushrooms: These mushrooms offer a rich, earthy flavor and a meaty texture that complements the Brussels sprouts beautifully. They hold their shape well during sautéing.
• Onion and Garlic: The aromatic base of any good casserole. Freshly chopped onion and minced garlic will provide a depth of savory flavor to the sauce.
• Butter: Used for sautéing the vegetables and creating the roux for the cream sauce, butter is key for richness and flavor.
• All-Purpose Flour: This is the thickening agent for our homemade cream sauce, forming a roux with the butter to create a smooth, velvety base.
• Chicken or Vegetable Broth: Provides liquid and flavor to the sauce. Use vegetable broth for a vegetarian-friendly option.
• Half and Half: Adds luxurious creaminess to the sauce without being overly heavy. You can substitute with whole milk for a lighter version or heavy cream for a richer one.
• Panko Breadcrumbs: These Japanese-style breadcrumbs are lighter and crispier than traditional breadcrumbs, providing an excellent crunch for the topping.
• French Fried Onions: The iconic crispy topping that provides a savory, oniony crunch and a hint of nostalgia.
• Salt and Pepper: Essential seasonings to enhance all the flavors in the casserole. Adjust to your personal taste.

Mastering the Brussels Sprouts and Mushroom Casserole: Step-by-Step Guide

Creating this delicious side dish is simpler than you might think, especially when you follow these easy steps. We’re building layers of flavor, ensuring every bite is a delightful experience. Remember to gather all your ingredients and prep them before you start cooking for a smooth process.

1. Sauté Brussels Sprouts for Golden Perfection: Begin by preheating your oven to 375°F (190°C) and greasing a 9×13 inch casserole dish. In a large skillet over medium heat, melt 2 tablespoons of butter. Add your halved Brussels sprouts, ensuring they are not overcrowded in the pan (cook in batches if necessary). Sauté them for about 5 minutes, stirring occasionally, until they develop a beautiful golden-brown caramelization on the cut sides. This crucial step deepens their flavor and softens them slightly while maintaining a pleasant bite. Transfer the cooked sprouts to a separate bowl and set aside.

2. Sauté Mushrooms and Onions to Tender Sweetness: Add another 2 tablespoons of butter to the same skillet. Once melted, introduce the sliced mushrooms and chopped onions. Season lightly with salt and pepper to draw out moisture and enhance their flavors. Cook for approximately 5-6 minutes, stirring periodically, until the mushrooms are tender and the onions have softened and become translucent. Finally, stir in the minced garlic and cook for just 1 minute more until fragrant. Be careful not to burn the garlic! Add this mixture to the bowl with the sautéed Brussels sprouts.

3. Craft a Velvety, Homemade Cream Sauce: Now it’s time to build the heart of our casserole: the rich, creamy sauce. In the same skillet (no need to clean it, the residual flavors add depth!), melt another 2 tablespoons of butter. Sprinkle in the all-purpose flour and whisk vigorously to create a smooth, thick paste, known as a roux. Cook this roux for 1-2 minutes to eliminate any raw flour taste. Gradually, while continuously whisking, pour in the chicken or vegetable broth, ensuring no lumps form. Then, slowly whisk in the half and half. Increase the heat to medium-high and bring the sauce to a gentle boil, still whisking frequently. Reduce the heat and simmer for about 5 minutes, or until the sauce has visibly thickened to a luxurious consistency. Season generously with salt and pepper to taste.

4. Assemble and Top Your Casserole: Gently combine the prepared cream sauce with the sautéed Brussels sprouts and mushroom mixture. Stir until all the vegetables are evenly coated in the rich, creamy sauce. Pour this delectable mixture into your greased 9×13 inch casserole dish, spreading it out evenly. For the crispy topping, melt the remaining 2 tablespoons of butter and combine it with the panko breadcrumbs in a medium bowl. Stir well until the breadcrumbs are lightly coated. Then, fold in the French fried onions. Generously scatter this crunchy topping over the Brussels sprouts and mushroom mixture in the baking dish. This combination ensures a perfect golden-brown, crispy crust.

5. Bake to Golden Perfection: Place the casserole dish in your preheated 375°F (190°C) oven. Bake for approximately 20-25 minutes, or until the casserole is bubbling around the edges and the topping has achieved a beautiful golden-brown hue and is wonderfully crispy. Keep an eye on it to prevent the topping from burning.

6. Serve and Savor: Once baked, remove the Brussels sprouts and mushroom casserole from the oven. Allow it to rest for a few minutes before serving. This resting period allows the sauce to set slightly, making it easier to scoop and ensuring all the flavors meld together perfectly. Serve hot and enjoy the compliments!

Planning Ahead: Can This Casserole Be Made in Advance?

Absolutely! The ability to prepare dishes ahead of time is a lifesaver during busy holidays like Thanksgiving. This Brussels sprouts and mushroom casserole is an excellent candidate for make-ahead cooking, allowing you to focus on other aspects of your meal on the big day.

To prepare this casserole in advance, simply follow all the steps through baking. Once the casserole is fully baked and the topping is golden and crispy, let it cool completely to room temperature. This is crucial for food safety and preventing condensation. Once cooled, cover the casserole dish tightly with plastic wrap and store it in the refrigerator for up to 2 days.

When you’re ready to reheat, remove the plastic wrap and cover the dish with aluminum foil. Warm it in a 350°F (175°C) oven for about 30 minutes. To regain that glorious crispy topping, remove the foil for the last 5-10 minutes of reheating. This will allow the French fried onions and panko to crisp up beautifully again, tasting as good as freshly baked.

For even easier day-of cooking, you can also prep your vegetables well in advance. Chop the Brussels sprouts (halving them), slice the mushrooms, and dice the onion up to 1 week ahead of time. Store each type of vegetable in separate airtight containers in the refrigerator until you’re ready to use them. This simple step can save a significant amount of time when you’re in the midst of holiday meal preparation.

Customizing Your Casserole: Delicious Variations and Substitutions

This Brussels sprouts and mushroom casserole is fantastic as is, but it’s also incredibly versatile! Feel free to experiment with these variations and substitutions to tailor it to your taste or dietary needs:

• Add Cheese for Extra Richness: Stir in ½ cup to 1 cup of shredded cheese (such as Gruyere, sharp cheddar, Parmesan, or a blend of Italian cheeses) into the cream sauce along with the vegetables before pouring it into the casserole dish. This will add another layer of savory flavor and gooey goodness.
• Spice It Up: For a little kick, add a pinch of red pepper flakes to the cream sauce while it simmers, or a dash of cayenne pepper for a warmer heat.
• Herbaceous Boost: Incorporate fresh herbs like chopped fresh thyme, rosemary, or sage into the cream sauce for an aromatic enhancement that perfectly complements the earthy flavors.
• Make it Gluten-Free: Easily convert this recipe to be gluten-free by using a 1:1 gluten-free all-purpose flour blend for the roux. Ensure your broth and French fried onions are also certified gluten-free.
• Vegetarian/Vegan Adaptations: For a strictly vegetarian dish, simply use vegetable broth. To make it vegan, swap butter for a plant-based butter alternative, half and half for a full-fat unsweetened plant-based milk or cream (like oat or cashew), and ensure your French fried onions are vegan (some brands are).
• Different Toppings: If French fried onions aren’t your preference, consider other crispy toppings. You could use only panko breadcrumbs, crushed butter crackers, or even crispy bacon bits for a smoky flavor.
• Add Other Vegetables: Feel free to toss in other quick-cooking vegetables like chopped bell peppers, spinach (stir in at the end until wilted), or even some diced cooked sweet potato for added color and nutrition.

Frequently Asked Questions (FAQ) About This Casserole

Here are answers to some common questions you might have about making this delicious Brussels sprouts and mushroom casserole:

Can I use frozen Brussels sprouts?

While fresh is preferred for the best texture and caramelization, you can use frozen Brussels sprouts. Thaw them completely and pat them very dry before sautéing to remove excess moisture. They might not brown as intensely, but they will still be delicious.

What kind of mushrooms are best for this recipe?

Cremini (baby bella) mushrooms are recommended for their earthy flavor and firm texture. White button mushrooms also work well. For a gourmet touch, you could use a mix of wild mushrooms.

Can I use regular milk instead of half and half?

Yes, you can. Whole milk will result in a slightly thinner and less rich sauce, but it will still be creamy and delicious. If you only have skim or 2% milk, you might want to add a tablespoon of cream cheese or a little extra flour to help thicken the sauce.

How long does the casserole last in the fridge?

Once baked and cooled, the casserole can be stored in an airtight container in the refrigerator for up to 3-4 days. Reheat according to the make-ahead instructions.

Can I freeze this casserole?

It’s generally not recommended to freeze casseroles with creamy sauces and crispy toppings, as the texture of both can suffer upon thawing and reheating. The sauce might separate, and the topping will lose its crispness. It’s best enjoyed fresh or made ahead and refrigerated for a couple of days.

Perfect Pairings: What to Serve with Your Casserole

This versatile Brussels sprouts and mushroom casserole isn’t just for Thanksgiving; it makes a fantastic side dish for many meals! It pairs beautifully with:

• Roast Turkey or Chicken: A natural fit for any holiday or Sunday dinner.
• Baked Ham or Pork Loin: The savory notes complement the sweetness of ham wonderfully.
• Steak or Roast Beef: A hearty vegetable side that stands up to rich meats.
• Vegetarian Main Courses: It can even be a star element of a vegetarian feast alongside other vegetable dishes.

More Festive Thanksgiving Side Dishes Favorites

• How to Make the Best Mashed Potatoes
• Easy Classic Stuffing
• Buttery Garlic Green Beans
• Roasted Sweet Potatoes with Maple Pecan Sauce

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This recipe was originally published November 2016.