Roux by John Spottiswood.

This is a thickener that's made from equal weights of flour and a fat, like butter or meat drippings. It's especially good for thickening rich, hearty stews and gravies. To make it, heat the fat in a pan, then gradually whisk in the flour. Cook the mixture, stirring constantly, for at least several minutes, then gradually whisk in the hot liquid you're trying to thicken. You must then cook the sauce for at least 30 minutes to prevent it from acquiring a grainy texture and a starchy, floury taste. Some cooks make large batches of roux, and store it in the refrigerator or freezer.