Preheat oven to 350°. Season turkey cavity with 1 tbsp. of the sage and salt and pepper to taste. Stuff breast cavity with 4 of the onion halves and the rear cavity with the remaining onions. Tie drumsticks together with kitchen twine. Season turkey all over with salt and pepper. Put turkey, breast side up, on a roasting rack in a heavy roasting pan. Add remaining sage to melted butter, then brush turkey all over with the seasoned butter. Add 1 cup water to roasting pan.
Roast turkey, basting it all over every 30 minutes or so with the seasoned butter, for about 15 minutes per pound or until skin is golden brown and internal temperature of turkey thigh registers 165° and breast registers about 180°, 3–4 hours. Transfer turkey to a carving board, loosely cover with foil, and let rest for 15 minutes. Remove twine, discard onions, and carve turkey. Serve with corn bread dressing on the side.