Step 1Wash the turkey inside and out, pat it dry and weigh it. Measure 1 tablespoon and one-half teaspoon salt into a bowl for every 5 pounds of turkey (for a 15-pound turkey, you'd have 3 1/2 tablespoons).
Step 2Lightly sprinkle the inside of the turkey with salt. Place the turkey on its back and salt the breasts, concentrating the salt in the center, where the meat is thickest. You'll probably use a little more than a tablespoon. It should look liberally seasoned, but not over-salted.
Step 3Turn the turkey on one side and sprinkle the entire side with salt, concentrating on the thigh. You should use about a tablespoon. Flip the turkey over and do the same with the opposite side.
Step 4Place the turkey in a 2 1/2 -gallon sealable plastic bag. Press out the air and seal tightly. Place the turkey, breast side up, in the refrigerator. Chill the turkey for three days: After the first day, redistribute the salt by opening the bag, massaging the salt into the bird and resealing the bag tightly, pressing out the air. After the second day, redistribute the salt again. Turn the turkey onto its breast to chill for the last day.
Step 5Remove the turkey from the bag and wipe it off with a paper towel. There should be no salt visible on the surface and the skin should be moist but not wet. Place the turkey, breast side up, on a plate and refrigerate uncovered for at least 8 hours.
Step 6On the day it is to be cooked, remove the turkey from the refrigerator and let rest at room temperature at least 1 hour. Meanwhile, heat the oven to 325 degrees.
Step 7Place the turkey breast-side down on a roasting rack in a roasting pan; put it in the oven. After 30 minutes, remove the pan from the oven and carefully turn the turkey over so the breast is facing up (it's easiest to do this by hand, using kitchen towels or oven mitts). Continue cooking an additional 1 1/2 hours, rotating the pan halfway through the roasting time.
Step 8Increase the oven temperature to 425 degrees, return the turkey to the oven and roast until a thermometer inserted in the deepest part of the thigh, but not touching the bone, reads 160 degrees, an additional 20 to 30 minutes. The total roasting time will be about 2 1/2 to 2 3/4 hours.
Step 9Remove the turkey from the oven, transfer it to a warm platter or carving board; tent loosely with foil. Let stand at least 30 minutes to let the juices redistribute through the meat. Carve and serve.