The cooked mixture of flour and fat used as a base for many Cajun dishes is roux.
A roux is made by cooking flour together with fat, commonly oil, butter, lard, or meat drippings. The mixture thickens sauces and stews while also contributing flavor. In Cajun and Creole cooking, roux is a foundational starting point for dishes such as gumbo, étouffée, and sauce piquante.
The color of a roux depends on how long it cooks. A pale roux retains more thickening power and has a mild flavor, while a medium or dark roux develops a toasted, nutty taste. Cajun cooks often use oil-based roux cooked to a medium or dark brown shade, because oil can withstand the prolonged cooking needed without burning as quickly as whole butter.
Roux is sometimes confused with a slurry, but the techniques are different. A slurry mixes flour or cornstarch with a cold liquid and is added later to thicken a dish; roux cooks the flour in fat first. Beurre manié is another related French technique, but it is an uncooked kneaded mixture of flour and butter. Roux also loses thickening strength as it becomes darker, trading structure for deeper flavor.