Melt two tablespoons butter in a pan. When it bubbles, add 2 tablespoons flour. Cook and stir about 1 minute to cook the flour. When it is cooked (don't let it brown!), add 1 cup milk and stir to blend. (The flour likes to stick around the edges, so get it out with a spoon.) It will take a while to thicken because it has been in the refrigerator. When it is thick, add about 1 cup grated cheese, if desired.