A roux is a mixture of flour and melted fat or oil that is used as the base for most gravies, sauces, and soups. It is generally made with butter and flour, although you can also substitute vegetable oil, lard, or other fats, so long as they are liquid. You can make a roux quite easily just by melting butter in a pan over medium heat, and whisking in an equal amount of all-purpose flour. To thicken a sauce with it, gradually add the sauce into the roux while beating with the whisk, allowing it to heat up a bit as you stir. When the mixture has reached the boiling point, the sauce has been thickened, and will become significantly thicker as it cools. If you are wondering what to do with a roux, read our pages on gravy and alfredo sauce.