In a small bowl combine the first five ingredients. With fingers, carefully loosen skin from the turkey; rub half the spice mixture under the skin.
Brush turkey with oil. Rub remaining spice mixture over turkey. Place lemons inside the neck and body cavity. Tuck wings under turkey; tie drumsticks together.
Arrange onions in a shallow roasting pan coated with cooking spray. Place turkey, breast side up, over onions.
Bake, uncovered, at 325° for 3-1/2 to 4 hours or until a thermometer inserted in thigh reads 180°, basting occasionally with pan drippings. Cover loosely with foil if turkey browns too quickly. Cover and let stand for 20 minutes before carving.
Remove lemons from cavity. When cool enough to handle, halve lemons and squeeze juice over turkey if desired. Discard onions.