In Louisiana Creole cooking, roux is the browned flour-and-fat mixture that thickens gumbo.
Roux is made by cooking flour with a fat such as butter, oil, or lard. The mixture may remain pale or be cooked to a darker color; Louisiana gumbo commonly uses a dark roux, whose toasted flavor is as important as its thickening power.
The technique reflects the French influence on Louisiana cuisine, while the darker style developed within the region’s Creole and Cajun cooking traditions. A cook must stir the mixture carefully because dark roux can burn quickly.
Roux is not the same as filé powder or okra, both of which can also help thicken gumbo. Gumbo recipes differ widely: some rely on roux, some combine it with okra or filé, and some use different proportions of seafood, poultry, and sausage.