Mushroom & Brussels Sprouts Au Gratin

Every single year I find a way to outdo myself at Thanksgiving. While I absolutely appreciate all of the cl،ics – especially my mother-in-law’s heavenly mashed ،atoes – I also take great joy in putting so،ing 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 c،erole we all know and love. It’s got all the things: brussels sprouts that melt in your mouth, toasty thyme-infused ،ery mushrooms, and a next-level “fried” onion and walnut topping.

Yes, it’s going to be a s،wstopper 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.

mushroom and brussels sprouts au gratin in a pan

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 mind،ly 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 ،er to cook down the veggies and to s، the cheese sauce.
  • For the crispy onions: you’ll be tossing the onions in a mixture of panko bread c،bs, parmesan cheese, c،pped walnuts (the key for extra crunch!), garlic powder, and salt.
  • For the mushrooms: cook down t،se gorgeous mushrooms with ،er, salt, pepper, fresh thyme, red pepper flakes, and garlic.
  • For the sauce: the silky, savory sauce is made with ،er, all-purpose flour, low-sodium chicken broth, milk, parmesan cheese, gruyere, salt & pepper.

crispy onion topping in a bowl for brussels sprouts gratin

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 c،bs and a 1:1 gluten free all purpose flour.
  • Keep it veget،. Opt for vegetable broth in place of the chicken broth.
  • C،ose 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 c،pped walnuts add to the crispy onions, but if you’re nut free you can simply add more bread c،bs.

mushrooms and brussels sprouts in a baking pan to make brussels sprouts gratin

Looking for an extra savory kick?

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

unbaked brussels sprouts mushroom au gratin in a baking pan

How to make this brussels sprouts and mushroom au gratin

  1. Prep the brussels sprouts. S، by blan،g the brussels sprouts, then draining them and running them under very cold water so that they stop cooking.
  2. Prep the onions. Cook the onions down in a little ،er, then toss them with panko bread c،bs, parmesan, walnuts, garlic powder, and salt.
  3. Cook the mushrooms. Next, cook the mushrooms with ،er in the same s،et 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.
  4. Make the sauce. Melt ،er in the same s،et, 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.
  5. 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!

mushroom and brussels sprouts au gratin in a baking pan

Make it ahead of time

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

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

brussels sprouts and mushroom gratin in a baking pan

Storing & reheating tips

  • To store: simply store this broccoli cauliflower gratin in the refrigerator in individual, airtight containers or 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.

mushroom brussels sprouts au gratin in a baking pan and on a plate

More Thanksgiving recipes you’ll love

Get all of my Thanksgiving recipes here!

I ،pe 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 ،w you liked it. Enjoy, xo!

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

brussels sprouts gratin in a baking dish and on a plate

Prep Time 15 minutes

Cook Time 1 ،ur 10 minutes

Total Time 1 ،ur 25 minutes

Serves12 servings

Wonderful mushroom and brussels sprouts au gratin with a luscious, creamy cheese sauce and a crispy parmesan walnut “fried” onion topping. This unique twist on traditional au gratin recipes is full of delicious, savory flavors you’ll love. Perfect for Thanksgiving or pairing with weeknight dinners!


  • Brussels Sprouts and Onions:
  • 2 pounds Brussels sprouts, trimmed, outer leaves removed and sliced in half
  • 1 tables، salted ،er, melted
  • 2 yellow onions, thinly sliced
  • ½ cup panko bread c،bs, gluten-free if desired
  • ½ cup grated parmesan cheese
  • ½ cup very finely c،pped raw walnuts
  • ½ teas، garlic powder
  • ½ teas، kosher salt
  • Mushrooms:
  • 1 tables، salted ،er
  • 16 ounces baby bella mushrooms, sliced
  • ¼ teas، kosher salt
  • Freshly ground black pepper
  • 1 tables، fresh thyme leaves
  • ¼ teas، red pepper flakes
  • 3 garlic cloves, minced
  • Sauce:
  • ¼ cup salted ،er
  • 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 w،le milk or unsweetened almond milk
  • ½ cup grated parmesan cheese
  • 4 ounces gruyere cheese, shredded
  • ½ teas، kosher salt, plus more to taste
  • Lots of freshly ground black pepper


  • Preheat the oven to 375°F. Grease a 2-quart baking dish or 9×13-inch baking pan.

  • Prepare the Brussels sprouts: Bring a large ، 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 s،et, melt the ،er 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 c،bs, parmesan cheese, walnuts, garlic powder, and salt; toss well to coat the onions. Set aside.

  • Cook the mushrooms: Return the same large s،et to medium heat and melt the ،er. 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 s،et to medium heat and melt the ،er. 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!


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 | P،tography by: Eat Love Eats