01 - In a medium saucepan, melt the butter over medium heat.
02 - Whisk in the flour and cook, stirring constantly, for 1 to 2 minutes until the mixture bubbles without browning.
03 - Gradually pour in the milk while whisking continuously to prevent lumps.
04 - Cook, whisking frequently, until the sauce thickens and coats the back of a spoon, about 5 to 7 minutes.
05 - Stir in salt, pepper, and nutmeg. If desired, add Parmesan cheese and whisk until fully melted and smooth.
06 - Taste and adjust seasoning if necessary. Serve immediately or keep warm over low heat.