It is worthwhile learning how to make a home-made white sauce based on a roux. Roux is simply the French derived word for a mixture of melted butter and white flour. It is the roux that thickens the milk. What happens is amazing and very useful.
Start with accurately weighed ingredients. 30g white plain flour with 30g butter (do not use low fat spread) Have ready 300ml milk.
Cooks know how:
Scientifically the starch grains are responsible for the thickening of a sauce. The starch absorbs and swells due to the liquid. As the heat is increased the starch grains burst and thicken the mixture. Uncooked starch grains taste raw and therefore it is important to boil the sauce to ensure all starch grains are cooked. White flour gelatinises around boiling point. A thickened sauce sets on cooling and returns to fluid on heating
Making a flavoured roux:
You can cook something such as finely chopped onions or mushrooms in the butter and then add the flour and create the roux sauce. Follow the same method as with a plain white sauce but allow the vegetable to cook (use about 15g more butter with mushrooms as they absorb the fat quickly)
Uses for a white sauce:
Cauliflower cheese, Lasagne, Fish and parsley sauce, Vol au vent, Pasta bake, Onions in cheese sauce.
Using a basic roux with different amounts of milk can give you a pouring sauce (400ml milk), a coating sauce (300ml as recipe) or a very thick sauce, binding sauce (200ml milk) to combine other ingredients for pastry fillings.
See recipe for Bechamel sauce.