Make the Roux: Melt the butter in a medium saucepan over medium heat. Stir in the flour and cook for 2-3 minutes, stirring constantly, until the mixture is lightly golden.
Add the Milk: Gradually whisk in the milk, making sure to avoid lumps. Continue whisking until the sauce thickens, about 5-7 minutes.
Add the Cheese: Remove the saucepan from heat and stir in the Gruyère and Parmesan cheese until fully melted and smooth.
Season the Sauce: Add salt, pepper, and nutmeg (if using), and whisk until the seasonings are fully incorporated.
Adjust Consistency: If the sauce is too thick, add a splash of milk or white wine to thin it out to your desired consistency.
Serve: Use the Mornay sauce immediately as a topping for vegetables, pasta, or baked dishes, or as a base for a rich mac and cheese.
