Roux 's a cooked mixture of wheat flour and fat, traditionally clarified butter. It's the thickening agent of four of the mother sauces of classical French cooking: sauce béchamel, sauce velouté, allemande sauce, and sauce espagnole. Butter, vegetable oils, or lard are common fats used. It's used as a thickener for gravy, other sauces, soups and stews. It's typically made equal parts of flour and fat by weight. Methods The fat's heated in a pot or pan, melting it if necessary, then th… (More on Roux)