1. Preheat your oven to 430°F. This is a crucial step to ensure that the cauliflower and mushrooms roast beautifully. The high temperature will help to caramelize the vegetables, enhancing their natural sweetness.
2. Toss the cauliflower florets and mushrooms in a bowl with olive oil, sea salt, black pepper, and garlic powder. Mix well to ensure every piece is coated in the seasoning. This step is essential for flavor development, so don’t rush it.
3. Spread the seasoned vegetables on a baking sheet in a single layer. This helps them roast evenly without steaming, which is key for achieving that desired golden-brown color. Bake in the preheated oven for 25 to 30 minutes until tender and lightly browned. Keep an eye on them and give a gentle stir halfway through for even roasting.
4. Once done, remove the vegetables from the oven and set aside to cool slightly. This pause allows you to prepare the cheese sauce without feeling rushed.
5. In a medium saucepan over medium heat, melt unsalted butter. As it melts, you’ll want to add all-purpose flour to create a roux. Whisk continuously for about one minute to cook out the raw flour taste.
6. Gradually whisk in whole milk. This step is crucial; if you pour it in too quickly, lumps may form. Keep whisking constantly until the mixture is smooth.
7. Cook the sauce for about 3 to 5 minutes, stirring frequently, until it thickens. The sauce should coat the back of a spoon when it’s ready.
8. Stir in sea salt, nutmeg powder, and both types of cheddar and Gruyère cheese. Keep stirring until all the cheese has melted and the sauce is creamy and smooth.
9. Reduce the oven temperature to 390°F. In a baking dish, combine the roasted cauliflower and mushrooms with the cheese sauce. Use a spatula to mix everything evenly, ensuring the vegetables are well coated.
10. Finally, top with the remaining grated cheddar and Gruyère cheese. This layer will create a deliciously crispy top once baked.
11. Bake for 15 to 20 minutes or until the top is bubbly and golden brown. Your kitchen will smell heavenly as it bakes!
12. Once finished, remove from the oven and let it sit for a few minutes before serving. This brief wait allows the casserole to set slightly, making it easier to serve.
