Roux (pronounced /ˈruː/) is a cooked mixture of wheat flour and fat, traditionally clarified butter. It is the thickening agent of three of the mother sauces of classical French cooking: sauce béchamel, sauce velouté and sauce espagnole.
Butter, vegetable oils, or lard are commonly used fats. It is used as a thickener for gravy, other sauces, soups and stews. It is typically made from equal parts of flour and fat by weight. When used in Italian food, roux is traditionally equal parts of butter and flour. By contrast, Hungarian cuisine uses lard (in its rendered form) or - more recently - vegetable oil instead of butter for the preparation of roux (which is called rántás in Hungarian).
The fat is heated in a pot or pan, melting it if necessary, then the flour is added. The mixture is stirred until the flour is incorporated, and then cooked until at least the point where a raw flour taste is no longer apparent and until desired color has been reached.
The final results can range from the nearly white to the nearly black, depending on the length of time it is over the heat, and its intended use. The end result is a thickening and flavoring agent.