This is a decadent dish that's perfect for the Thanksgiving table. Creamy and flavorful cheese sauce poured over roasted cauliflower forms a perfect pairing. It's quick and can be created early and baked right before serving, which is great for hectic holiday preparations.
Preheat the oven to 425°F. Butter a 9x13 or 8x11 baking dish and set it aside.
Clean and cut the cauliflower into bite-sized florets. Spread the florets on two parchment-lined baking sheets. Drizzle with 2 tbsp olive oil, a pinch of kosher salt, and black pepper. Roast in the preheated oven until the cauliflower is tender but not soft and beginning to brown, 12-18 minutes. Once slightly tender, remove the cauliflower from the oven and pour it into the prepared baking dish. Reduce the oven to 375°F.
Melt 2 tbsp of the butter in a saucepan over medium heat. Once melted, add the onion. Sauté the onion until it is translucent but not brown, 4-6 minutes. Add the garlic and sauté until it is fragrant, 1 minute. Season with the kosher salt and black pepper.
Sprinkle in the flour. Stir and cook the flour until it is a paste coating the pan, 1-2 minutes. Using a whisk, slowly add the milk, about ¼ cup at a time, making sure to whisk it smooth before adding more. Once all the milk is added, bring the mixture to a simmer and cook until it is thickened, about 1-2 minutes. Remove from the heat.
Add the nutmeg, Dijon mustard, ¾ cup Gruyère cheese, and Parmesan cheese. Stir until smooth. Pour the sauce over the prepared cauliflower. Sprinkle with the remaining Gruyère and Parmesan cheese. In a bowl, melt the remaining tbsp of butter. Mix the butter with the breadcrumbs and sprinkle them over the cauliflower.
Bake in the 375°F oven until the breadcrumbs are toasted and brown, 25-30 minutes. Remove from the oven and serve.
When breaking down the cauliflower, pieces of the stem can also be included in the dish. However, keep the core out. But while it won't work well in this dish, it can still be eaten. Peel off some of the outer layers of the core, and you have the perfect snack!When adding the garlic to the sauce, make sure not to overcook it. The minced garlic is small, so it cooks quickly; it just needs a few seconds to warm. If it gets overcooked, the taste will become bitter.When adding the milk, stir it in ¼ to ½ cups at a time. This extra time creates a smoother sauce in the end.