These questions refer to the fat and flour mixture used to thicken sauces.
When flour is cooked in a fat, the resulting product can be used to thicken sauces. Questions in this category most often address variations on the proportions, technique tips, or problems in using the resulting roux.
Often these questions compare the use of a roux to other thickening methods such as a cornstarch slurry (mix with water) or arrowroot.
