Pour all drippings from the roasting pan into a measuring cup and set aside to allow fat to rise to top, about 20 minutes. Pour off the fat into a small bowl and reserve. Measure remaining juices. You need 1 cup. If less, augment the juices with turkey or chicken stock.
Put the roasting pan on the top of the stove so it spans two burners, and turn the burners to medium low. Spoon 1/2 cup of turkey fat back into the pan (substitute butter if you don't have enough turkey fat). Whisk flour and cornstarch into fat. Cook, whisking, until flour is deeply browned and has a nutty aroma.
Whisking constantly, pour the turkey stock and 1 cup reserved turkey juices into the pan. Whisk 1 minute, then use a wooden spoon to dislodge the browned bits from the bottom of the roasting pan. Resume whisking until gravy is smooth, then turn heat to high and bring to a boil.
Boil until gravy is thick enough to coat the back of a metal spoon, 3 to 5 minutes. Season to taste with salt and pepper. Pour through a fine strainer into a gravy boat and serve immediately.
Note: Nutritional analysis is per cup.