Method
Make a roux with the butter and flour. Draw pot away from heat and add a little milk and stir it well. When the mixture is smooth, add a little more milk and stir until smooth. Continue adding the milk until smooth. Continue adding the milk until it is used up. Bring to the boil, stirring all the time and when thick enough remove from heat. Beat well to remove all lumps and to give a gloss to a sauce.If there are any lumps use a wire whisk to break them up. For a coating sauce, use less milk to make a thicker sauce.