Bechamel is a classic French white sauce made with a roux of butter and flour with milk added to form a rich sauce. A basic medium sauce is a ratio of 2/2/1, 2 tablespoons flour, 2 tablespoons butter and 1 cup of milk.
Heat the butter in a sauce pan, add the flour and cook over medium heat until it just starts to change color but don't allow it to brown. Stir in the milk. Cook until it thickens.