01 - In a medium saucepan, melt the butter over medium heat.
02 - Whisk in the flour and cook for 1 to 2 minutes, stirring constantly, until bubbling but not browned.
03 - Gradually pour in the milk, whisking continuously to avoid lumps.
04 - Bring sauce to a gentle simmer and cook for 3 to 4 minutes until thickened.
05 - Reduce heat to low, then stir in cheddar and Parmesan until fully melted and smooth.
06 - Add salt, white pepper, and nutmeg if desired, mixing evenly.
07 - Serve immediately or keep warm over low heat, stirring occasionally.