All-in-One White Sauce Ingredients: 600ml milk, 50g butter or margarine, 50g plain flour, 1 x 5ml teaspoon mustard paste. 1. Put the milk, flour, butter and mustard paste in a saucepan. Use a balloon whisk or wooden spoon and stir ALL THE TIME over a medium heat until the sauce boils and thickens. 2. Continue cooking for about 1 minute to make sure the flour is completely cooked. The finished sauce should be thick enough to coat the back of a spoon and should be smooth and glossy. Add salt and pepper to taste. 3. If you are making a cheese sauce, add the cheese off the heat when the sauce is ready. Stir well and the cheese will melt without needing to go back on the heat. If you overcook the cheese, it will be stringy and indigestible.