A roux is a mixture of flour and fat that is cooked into a paste and used to thicken sauces, gravy, curries, and stews. Butter is most commonly used, but bacon grease or lard can also be used. Roux that is cooked longer will gradually deepen in color through the Blond stage and finally to the Brown stage. Béchamel sauce uses a white roux, which is cooked just long enough to thicken before it can begin browning.