Prepare the Béchamel Sauce: In a small saucepan over medium heat, melt 2 tbsp of the unsalted butter. Whisk in the all-purpose flour and cook for 1 minute, stirring constantly, to form a roux. Gradually whisk in the warm milk until the mixture is completely smooth. Bring to a simmer and cook, whisking often, until the sauce thickens enough to coat the back of a spoon, about 5-7 minutes. Remove from heat and stir in the nutmeg, salt, pepper, and 1/2 cup of the shredded Gruyère cheese until melted and smooth. Set aside.