A fresh 12- to 16-pound turkey might need some time to come to room temperature. A frozen turkey of the same size will take 6 to 9 hours in cold water or 2 to 3 days in the refrigerator to thaw. Preheat the oven to 325 degrees. Rinse the turkey inside and out with cold running water and pat dry. Sprinkle the seasonings and ground pepper in the cavity.
Add the onions wedges. Place the turkey breast-side-down in a large roasting pan. The cooking time for a 12- to 16-pound turkey is about 3 to 4 hours. Halfway through the cooking time, turn the turkey breast-side-up and insert a meat thermometer. Continue cooking until the meat thermometer registers 180 to 185 degrees. Let the turkey stand for 15 to 20 minutes before carving.