Trussing gives a turkey (or any other bird) a smooth, compact shape so it will cook evenly and retain moisture.
1. Remove outer packaging and the packages containing the giblets and neck from the body and neck cavities (you may need to release the band of skin or the wire or plastic lock from the legs before the neck can be removed). Rinse the body and neck cavities and the outside of the turkey with cold, running water, and pat dry. Trim excess fat.
2. Massage the legs to make the skin pliable. When you secure the legs, tuck the excess skin between the breast and the legs so the skin won't split as the turkey cooks. Secure the legs.
3. Lift the tips of the wings up and over the breast to the back, and tuck them under the turkey; secure the excess skin of the neck flap (if necessary, use a wooden pick).
Note: The wishbone can be removed before cooking to make carving the breast meat easier. Pull the neck flap back, and feel along the breast until you locate the wishbone. Using a sharp, narrow-blade knife, cut along both sides of the wishbone and around the top edge. Place your fingers around the wishbone, and pull it loose.
4. Line the bottom of a shallow roasting pan with aluminum foil for easier cleanup. Place turkey on chopped onion, if desired; coat with cooking spray. Insert a meat thermometer in the meaty part of the thigh, making sure it doesn't touch the bone. Check the internal temperature of the turkey at the beginning of the last hour of the cooking time so you don't overcook it. If the turkey is browning too fast, make a tent of aluminum foil to shield the breast. After removing the turkey from the oven, cut a small slit between the thigh and the rib cage to release the juices. Let stand 10 minutes before carving.