In a medium saucepan over medium heat, melt 2 tablespoons of butter until bubbly but not browned. Stir in 2 tablespoons of all-purpose flour, mixing well to create a smooth paste. Cook this roux for about 2-3 minutes, stirring continuously until it turns a light golden color, indicating it's ready for the next step.

Gradually whisk in 2 cups of whole milk, ensuring to break up any lumps formed by the roux. Continue stirring constantly as you bring the mixture to a gentle simmer over medium heat. Cook for 5-7 minutes until the sauce thickens enough to coat the back of a spoon.

Once thickened, remove the saucepan from the heat. Stir in 1/2 cup of grated Gruyère cheese and 1/4 cup of grated Parmesan cheese, mixing until completely melted and the sauce is velvety smooth.

Add a pinch of salt, freshly cracked black pepper, and optional ground nutmeg or mustard powder to taste. Adjust its consistency with a splash of milk or white wine, stirring until smooth.

Use the Mornay sauce immediately while it's steaming and warm. Drizzle over your favorite dishes, or store in an airtight container in the refrigerator.