Begin by cutting a fresh head of cauliflower into small florets. Steam or boil the florets in a pot of salted water for about 5 to 6 minutes until they are firm yet tender. Drain thoroughly.

Preheat your oven to 375°F (190°C) and grease an 8x11x2-inch casserole dish with unsalted butter.

In a medium saucepan, melt 2 tablespoons of unsalted butter over medium heat. Add 2 tablespoons of all-purpose flour to create a roux, whisking continuously for about 2-3 minutes until it turns a light golden color. Gradually pour in 2 cups of milk, whisking until thickened, about 4-5 minutes. Season with fresh thyme, nutmeg, kosher salt, and black pepper to taste.

Remove the béchamel sauce from heat and stir in ½ cup of grated Parmesan and ½ cup of Gruyère cheese until completely melted.

Spread a thin layer of the cheese sauce in the prepared casserole dish, then add the drained cauliflower florets in an even layer. Pour the remaining cheese sauce over the top, ensuring every piece is coated, and sprinkle fresh breadcrumbs and remaining Gruyère cheese on top.

Bake for 30 minutes until the top is golden brown and bubbly. Serve hot or at room temperature.