Heat butter in a medium sauce pan, until melted.
Add flour and stir until smooth. Cook over medium heat until light golden brown, about 6 to 7 minutes.
Meanwhile, heat the milk in separate pan until just about to boil.
Add warmed milk to butter mixture 1 cup at a time, whisking continuously until very smooth.
Bring mixture to a boil and continue cooking for 30 seconds. Remove pan from the heat. Season with salt and nutmeg and set aside.