The French term for a thickening mixture of flour and fat cooked together is roux.
A roux is made by stirring flour into melted fat and cooking the mixture before adding liquid. The fat disperses the flour, while cooking removes the raw flour taste and develops flavour. When liquid is whisked in, the flour’s starch thickens sauces, gravies, soups and stews. Recipes commonly use roughly equal weights of flour and fat, though proportions vary.
Classical French cooking recognizes white, blond and brown roux, with colour determined by cooking time. White roux contributes little flavour and is used for sauces such as béchamel. Darker roux develops nutty, roasted notes but has less thickening power because prolonged heating reduces the starch’s effectiveness.
Roux is not the same as beurre manié, a kneaded uncooked mixture of butter and flour added to a hot liquid near the end of cooking. A slurry, another common alternative, mixes starch with cold liquid rather than fat. Roux also appears beyond French cuisine, including the dark oil- or lard-based roux used in Cajun and Creole dishes such as gumbo.