The most common way of thickening gravy is by using cornstarch, flour or another starch. Starches thicken gravy by absorbing liquid during cooking. Cooks can also thicken gravy by allowing liquid to evaporate from it, a process called reduction.
Because adding starch directly to hot gravy creates lumps, cooks first create a slurry or roux containing the starch. These processes separate the granules within the starch so that they expand individually instead of in a clump when the cook adds them to the gravy.
To thicken a gravy with cornstarch, a cook begins by mixing 2 to 2 1/2 teaspoons of cornstarch per cup of gravy to be thickened with a small amount of cold water or stock. The cook then whisks the resulting slurry into the gravy.
If the cook chooses to use flour instead of cornstarch, she can slowly cook 2 tablespoons of flour per cup of gravy to be thickened with butter or pan drippings to create a roux. This method both separates the flour granules and eliminates the taste of raw flour. The cook should allow the roux to cool slightly before whisking it into the gravy.
To thicken a gravy by reduction, the cook simmers the gravy in an uncovered pan, whisking as needed to break up lumps. When the gravy's level drops to halfway between its original level and the bottom of the pan, the cook removes the gravy from the heat. This method creates a more intense flavor than using starches.