To make the sauce, in a heavy saucepan melt the butter and heat WITHOUT browning. Using a wire whisk, stir in the flour. Cook for 1 to 2 minutes over low heat, then remove from the burner and gradually stir in the warm milk. Move the pan back to the heat and bring to a boil, stirring steadily for a smooth sauce. Add a pinch of salt and white pepper, and grate a little nutmeg for a nice flavor. As a general rule, thin white sauce is used for soups and sauces, medium for pastitsio and moussaka and other au gratin dishes, and a thick sauce for croquettes and souffles.