01 - Place butter in a medium saucepan over medium heat and melt completely.
02 - Whisk in flour continuously for 1 to 2 minutes until the mixture thickens and turns lightly golden.
03 - Gradually whisk in milk, stirring constantly to prevent lumps, and cook until the sauce thickens, approximately 3 to 5 minutes.
04 - Reduce heat to low and add cheddar, mozzarella, and Parmesan cheeses. Stir until fully melted and the sauce is smooth.
05 - Add salt, black pepper, nutmeg, and mustard powder if desired. Taste and adjust seasoning accordingly.
06 - Serve the sauce immediately over preferred dishes or keep warm on low heat, stirring occasionally.