Bring a large pot of salted water to a boil. Add in the cauliflower florets and boil for 4 to 5 minutes. Drain and set the cauliflower aside.
Melt 2 tablespoons butter in a sauce pan over medium-low heat. Add the flour, whisking until combined. Add milk and whisk while the mixture comes to a boil and thickens.
Remove the pan from heat and add 1 teaspoon of salt, 1 teaspoon of pepper, and 1 cup shredded of shredded Gruyere cheese. Stir to combine.
Pour ½ cup of sauce on the bottom of a deep baking dish. Add the cauliflower to the baking dish, then top with remaining cheese sauce. Top with ½ cup grated of Parmesan cheese.