A roux is a mixture of equal parts fat and flour, cooked together and used to thicken sauces, soups, and stews. The name comes from the French word for "reddish-brown," describing the color a longer-cooked roux takes on. Butter or oil is melted, flour is whisked in, and the mixture is stirred over heat until the raw flour taste cooks out -- a few minutes for a light roux, up to an hour for a dark one. Light roux (white or blond) is used for pale sauces like bechamel and veloute; darker roux gives espagnole its depth and is cooked longest for Cajun and Creole dishes like gumbo and etouffee, where it also adds a nutty, toasted flavor. Roux can be made ahead and reheated before use.