In a saucepan melt the butter over medium heat. Add the flour and cook, stirring constantly, until the sauce is frothy, about 1 minute. Do not allow it to go brown. 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. Simmer for another 2 to 3 minutes.

This is now called a Bechamel sauce.

If you want to make Mornay sauce stir in the cheese and whisk until melted. If the sauce seems to thick, thin with a little milk.