01 - In a medium saucepan, melt the butter over medium heat.
02 - Whisk in the flour and cook, stirring constantly, until the mixture bubbles but does not brown, about 1 to 2 minutes.
03 - Gradually whisk in warm milk, avoiding lumps; continue cooking and stirring frequently until sauce thickens, approximately 3 to 4 minutes.
04 - Lower heat to low, then add cheddar, Parmesan, and cream cheese; stir until fully melted and sauce becomes smooth.
05 - Season with salt, pepper, and optional nutmeg; taste and adjust seasonings as preferred.
06 - Remove from heat and immediately pour over cooked pasta; toss to coat evenly.