Roux is the darkened flour-and-fat mixture used to thicken many Louisiana dishes.
A roux is made by cooking flour with fat, commonly butter, oil, or rendered meat fat. The mixture thickens liquid because cooked flour starch absorbs water, but Louisiana cooks also value the toasted flavor created as the roux darkens.
Cajun and Creole recipes may use roux in gumbo, étouffée, and other gravies. A dark roux contributes a deep, nutty taste, though it thickens less effectively than a pale roux because prolonged cooking changes the starch.
Making a dark roux requires attention because it can burn quickly. Once burned, it usually cannot be rescued and must be started again. Gumbo recipes differ over whether the roux should be blond, brown, or nearly chocolate-colored, and some gumbos use okra or filé powder as additional thickeners.