Step 1

Preheat the oven to 375°F. Grease a 2-quart baking dish or 9x13-inch baking pan.

Step 2

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.

Step 3

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.

Step 4

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.

Step 5

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.

Step 6

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.