Mushroom & Brussels Sprouts Au Gratin | Ambitious Kitchen (2024)

Every single year I find a way to outdo myself at Thanksgiving. While I absolutely appreciate all of the classics – especially my mother-in-law’s heavenly mashed potatoes – I also take great joy in putting something new and different on the table. See for yourself in my ultimate Thanksgiving roundup!

This stunning mushroom and brussels sprouts au gratin is an upgraded version of the green bean casserole we all know and love. It’s got all the things: brussels sprouts that melt in your mouth, toasty thyme-infused buttery mushrooms, and a next-level “fried” onion and walnut topping.

Yes, it’s going to be a showstopper on your Thanksgiving table, but don’t wait for Thanksgiving to try it! It would be a great pairing with your Sunday night roasted chicken. My gosh, you’re going to love it! Byeeeee green beans.

Ingredients in this mushroom and brussels sprouts au gratin

Tender brussels sprouts, savory mushrooms, crispy onions, and a creamy parmesan-gruyere cheese sauce — this au gratin recipe is mindblowly delicious. We’re skipping the cream of mushroom soup here, people! Here’s what you’ll need to make it:

Veggies: you’ll need fresh brussels sprouts, yellow onions, and baby bella mushrooms for this au gratin recipe.
Butter: we’re adding salted butter to cook down the veggies and to start the cheese sauce.
For the crispy onions: you’ll be tossing the onions in a mixture of panko bread crumbs, parmesan cheese, chopped walnuts (the key for extra crunch!), garlic powder, and salt.
For the mushrooms: cook down those gorgeous mushrooms with butter, salt, pepper, fresh thyme, red pepper flakes, and garlic.
For the sauce: the silky, savory sauce is made with butter, all-purpose flour, low-sodium chicken broth, milk, parmesan cheese, gruyere, salt & pepper.

Customize this brussels sprouts gratin

Yes, there are easy ways to make this au gratin your own! Here’s what I can recommend:

Make it gluten-free. Simply use gluten free bread crumbs and a 1:1 gluten free all purpose flour.
Keep it vegetarian. Opt for vegetable broth in place of the chicken broth.
Choose your cheese. I love the combo of parmesan and gruyere cheese, but you could also use cheddar, fontina, or a smoky gouda!
Go nut free. I love the extra crispiness that chopped walnuts add to the crispy onions, but if you’re nut free you can simply add more bread crumbs.

Looking for an extra savory kick?

If you’re not serving vegetarians I suggest adding cooked bacon or ham for extra delicious savory flavor!

How to make this brussels sprouts and mushroom au gratin

Prep the brussels sprouts. Start by blanching the brussels sprouts, then draining them and running them under very cold water so that they stop cooking.
Prep the onions. Cook the onions down in a little butter, then toss them with panko bread crumbs, parmesan, walnuts, garlic powder, and salt.
Cook the mushrooms. Next, cook the mushrooms with butter in the same skillet the onions were in until they’re nice and golden. Then stir in the thyme, red pepper flakes, and garlic. Add them to the baking pan with the brussels sprouts.
Make the sauce. Melt butter in the same skillet, whisk in the flour until a paste forms, then slowly stream in the broth and milk and whisk until well combined. Bring the mixture to a boil, then reduce to a simmer and let it thicken to a gravy texture. Remove it from the heat and stir in the parmesan, gruyere, salt & pepper. Taste and adjust the seasonings.
Pour, top & bake. Pour the cheese sauce into the pan with the brussels sprouts and mushrooms and stir to coat everything. Evenly top the pan with the onions, bake it up until golden brown and bubbly, and enjoy!

Make it ahead of time

You can easily make this brussels sprouts gratin ahead to save time during Thanksgiving. Here’s what I recommend:

Prepare the recipe as directed but do not bake it. Wrap the casserole dish in foil and place it in the fridge up to 2 days ahead.
Once ready, remove the foil and bake the au gratin until the onions are nice and golden brown and the sauce is bubbly.
Serve immediately with all your favorite Thanksgiving dishes!

Storing & reheating tips

To store:simply store this broccoli cauliflower gratin in the refrigerator in individual,airtight containersor cover the entire pan and place it in the refrigerator. It will stay good for up to 5 days.
To reheat:reheat individual portions in the microwave or place the pan back in the oven until heated through.

More Thanksgiving recipes you’ll love

Parmesan Bacon Sourdough Mushroom Stuffing
Lightened Up Healthy Sweet Potato Casserole with Pecan Oat Streusel
The Best Creamy Garlic Slow Cooker Mashed Potatoes
Caramelized Garlic Carrots with Spicy Cilantro Yogurt Sauce

Get all of my Thanksgiving recipes here!

I hope you love this mushroom and brussels sprouts au gratin! If you make it be sure to leave a comment and a rating so I know how you liked it. Enjoy, xo!

Mushroom & Brussels Sprouts Au Gratin | Ambitious Kitchen (8)

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Mushroom & Brussels Sprouts Au Gratin with Crispy Parmesan Walnut Panko ‘Fried’ Onion

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Ingredients

Brussels Sprouts and Onions:
2 pounds Brussels sprouts, trimmed, outer leaves removed and sliced in half
1 tablespoon salted butter, melted
2 yellow onions, thinly sliced
½ cup panko bread crumbs, gluten-free if desired
½ cup grated parmesan cheese
½ cup very finely chopped raw walnuts
½ teaspoon garlic powder
½ teaspoon kosher salt
Mushrooms:
1 tablespoon salted butter
16 ounces baby bella mushrooms, sliced
¼ teaspoon kosher salt
Freshly ground black pepper
1 tablespoon fresh thyme leaves
¼ teaspoon red pepper flakes
3 garlic cloves, minced
Sauce:
¼ cup salted butter
⅓ cup all-purpose flour (or 1:1 gluten-free all-purpose flour)
1 ½ cups low-sodium chicken broth (or vegetable broth), plus another ½ cup more, if necessary
1 cup whole milk or unsweetened almond milk
½ cup grated parmesan cheese
4 ounces gruyere cheese, shredded
½ teaspoon kosher salt, plus more to taste
Lots of freshly ground black pepper

Instructions

Preheat the oven to 375°F. Grease a 2-quart baking dish or 9x13-inch baking pan.
Prepare the Brussels sprouts: Bring a large pot of generously salted water to a boil over high heat. Add the brussels sprouts and blanch, 2 minutes. Drain brussels and run under VERY cold water to stop the cooking process. Set aside.
Prepare the onions: In a large skillet, melt the butter over medium heat. Add the onions and cook until softened, about 8 minutes. Transfer the onions to a large bowl and add the panko bread crumbs, parmesan cheese, walnuts, garlic powder, and salt; toss well to coat the onions. Set aside.
Cook the mushrooms: Return the same large skillet to medium heat and melt the butter. Add the mushrooms, salt, and pepper. Cook mushrooms, stirring occasionally, until all liquid is absorbed from mushrooms and they are golden brown; 8 to 10 minutes. Stir in the thyme, red pepper flakes and garlic and cook, stirring until fragrant, 1 minute. Transfer to the baking pan with the Brussels sprouts and toss to combine.
Prepare the sauce: Return the same large skillet to medium heat and melt the butter. Whisk in the flour and cook until a paste forms, then slowly stream in the broth and then the milk, constantly whisking away any lumps, until smooth and well-combined. Increase heat and bring mixture to a boil, then reduce the heat to medium-low and simmer, whisking occasionally, until sauce thickens to the consistency of gravy, 5 minutes. (If the sauce gets too thick, you can add up to ½ cup more broth.) Remove from heat and stir in the parmesan, gruyere, salt, and pepper. Taste and adjust seasoning if necessary.
Pour the sauce over the Brussels and mushrooms and give it a gentle stir to coat the brussels and mushrooms. Evenly distribute the onion mixture over the top. Bake, uncovered, until the topping is golden brown and the sauce is bubbly, 25 to 35 minutes.

Recipe Notes

See the full post for tips, tricks, and great ways to customize this recipe!

Nutrition

Serving: 1serving (based on 12)Calories: 226calCarbohydrates: 16.1gProtein: 11.1gFat: 14.5gSaturated Fat: 6.3gFiber: 4.1gSugar: 3.6g

Recipe by: Monique Volz // Ambitious Kitchen | Photography by:Eat Love Eats