In a small saucepan melt the butter over medium-high heat. Add the flour and cook, stirring constantly, until the roux is pale yellow and frothy, about 1 minute. Do not allow the roux to brown. Slowly whisk in the milk and continue to whisk until the sauce thickens and comes to a boil, about 2 to 3 minutes. Reduce the heat to a simmer and season with the salt, pepper and nutmeg. Allow to simmer for 2 to 3 minutes. Stir in the cheese and whisk until melted. If the sauce seems too thick, thin with a little milk.
Use sauce immediately or refrigerate, surface covered with plastic wrap, for several days in the refrigerator.
Yield: about 1 cup