01 - In a medium saucepan over medium heat, melt the butter.
02 - Add the flour and whisk continuously for 1 to 2 minutes, ensuring it does not brown.
03 - Slowly pour in the milk while whisking constantly to prevent lumps. Continue whisking until the mixture thickens, about 3 to 4 minutes.
04 - Lower the heat to low. Gradually add the cheddar, mozzarella, and Parmesan, stirring after each addition until fully melted and smooth.
05 - Stir in salt, black pepper, ground mustard, and any optional add-ins. Adjust seasoning to taste.
06 - Remove from heat and serve immediately over preferred dishes.