A roux (French for russet brown) is a mixture of equal volumes of flour, and fat (usually butter or oil) which is used to thicken béchamel, velouté and brown sauces. For béchamel, butter is heated in a heavy-based pan until foaming, then flour is whisked in and cooked, also until foaming, before the milk is added. For velouté the roux is cooked longer—some cooks like it to be straw-colored (Fr. blond)—before adding the stock. For espagnole, the roux is cooked a deep russet brown for extra color and flavor.