Preheat oven to 325 degrees F. Remove neck and giblets from body and neck cavities; set aside (discard liver). Rinse and dry turkey with paper towels. Mix the onion, celery, garlic, and parsley together and fill the body cavity of the turkey with it. Place turkey, breast side up, on flat rack in shallow roasting pan. Coat skin with cooking spray.
Roast turkey 3 1/2 to 3 3/4 hours or until it is 180 degrees F in the thigh area. Cover breast and top of drumsticks with foil after 1 1/2 hours to prevent overcooking of breast.