In a small bowl, combine garlic, zests, parsley, juice, and oil and set aside.
Thoroughly rinse and pat dry the chickens. Generously rub the mixture inside and outside of chicken. Place onion and/or lemon inside the cavity. Place chickens, breast side up on a rack in a shallow roasting pan. Tuck wings under chicken and tie drumsticks together.
Bake uncovered for 2 to 2 1/2 hours or until a thermometer inserted reads 180 degrees. Baste occasionally with pan drippings. (Cover loosely with foil if chicken browns too quickly). Cover and let stand for 10 minutes before carving.