A good recipe for cooking a 20-pound turkey is the perfect roast turkey recipe developed by MarthaStewart.com. The recipe calls for two medium onions, two medium leeks, two carrots, two celery stalks, two dried bay leaves, three sprigs each of thyme and parsley and freshly ground pepper.
Brine the turkey for at least 24 hours before roasting it. Make the brine by combining 5 cups of sugar, 10 cups of water and 2 teaspoons of black whole peppercorns with the chopped onions, leeks, carrots and celery, bay leaves, thyme and parsley. The brine pot should be big enough to hold the entire turkey. Boil the brine mixture until the salt and sugar have completely dissolved. Add the turkey to the brine, put it in the refrigerator and let it soak for 24 hours.
Preheat the oven to 425 degrees. The cooking time for a 20 pound turkey is approximately 3 1/2 to 4 hours. Sprinkle 1 teaspoon each of salt and pepper into the turkey's cavity and place it in a roasting pan with the breast side up. The turkey should not touch the bottom of the pan. Use a roasting rack or place vegetables under the turkey to elevate it. Brush the entire turkey with butter and sprinkle more salt and pepper on top. Roast the turkey at 350 degrees until the internal temperature at the center of the turkey reads 165 degrees.