In a medium saucepan, melt 2 tablespoons of unsalted butter over medium heat until bubbly, about 1-2 minutes. Stir in 2 tablespoons of all-purpose flour, whisking constantly for 2-3 minutes until it turns lightly golden and has a nutty aroma.

Gradually whisk in 2 cups of whole milk, taking your time to prevent lumps. Increase the heat slightly and continue whisking until the mixture thickens, about 5-7 minutes.

Remove the saucepan from heat, and begin mixing in 1 cup of shredded Gruyère cheese and ½ cup of grated Parmesan cheese. Stir until fully melted and the sauce is smooth.

Sprinkle in salt and pepper to taste, along with a pinch of ground nutmeg and some mustard powder if using. Stir well to combine.

If your Mornay sauce is thicker than desired, add a little splash of milk or white wine to reach your desired consistency.

Once your Mornay sauce reaches the perfect consistency, use it right away over pasta, vegetables, or as a base for mac and cheese.