Roux – Melted fat (usually butter) to which flour has been added. A roux is used as a thickener for sauces and soups including three of the mother sauces of classical French cooking: béchamel sauce, velouté sauce, and espagnole sauce. This step is the foundation to developing a wide variety of dishes from gumbo to Basque Salsa Verde , scalloped potatoes, and cream of anything soup.