01 - In a medium saucepan over medium heat, melt the unsalted butter.
02 - Whisk in the all-purpose flour continuously for 1 to 2 minutes without browning to form a roux.
03 - Gradually pour in whole milk while whisking to prevent lumps, then continue whisking until the mixture thickens, about 3 to 4 minutes.
04 - Lower heat to low and add sharp cheddar, mozzarella, and Parmesan cheeses gradually, stirring after each addition until the sauce is fully melted and smooth.
05 - Stir in salt, ground black pepper, ground mustard, cayenne pepper, and Dijon mustard if using. Adjust seasoning according to taste.
06 - Remove from heat and serve immediately over pasta, vegetables, or your preferred dish.