Bring a large pot of salted water to a boil. Add the cauliflower and cook for about 5-6 minutes, until tender but firm. Drain and set aside.

Preheat the oven to 375°F (190°C) and grease an 8x11x2-inch casserole dish with butter or cooking spray.

In a medium saucepan, melt 4 tablespoons of butter over medium heat. Add 4 tablespoons of flour, whisking continuously to combine into a roux and cook for about 2 minutes.

Gradually pour in 2 cups of milk while whisking to prevent lumps. Continue stirring until the mixture thickens to a heavy cream consistency.

Stir in a pinch of salt, black pepper, nutmeg, thyme, and both cheeses until melted and combined to create a cheese sauce.

In the prepared dish, layer half of the cheese sauce at the bottom, add the cauliflower florets, then top with remaining cheese sauce.

Sprinkle fresh breadcrumbs over the top and the remaining Gruyère cheese to create a golden, crispy topping.

Bake for approximately 30 minutes or until golden brown and bubbling. Enjoy fresh from the oven or at room temperature.