Position rack in bottom third of oven and preheat to 425°F Toss onions and 2 tablespoons oil in large roasting pan. Roast until onions are golden brown, stirring every 15 minutes, about 1 hour.
Meanwhile, rinse turkey inside and out; pat dry. Slide hand under skin of turkey breast to loosen skin. Spread butter under skin over breast meat. Arrange 4 sage leaves under skin on each side of breast. If stuffing turkey, spoon stuffing loosely into main cavity. Tuck wing tips under turkey; tie legs together loosely. Rub turkey all over with 1 tablespoon oil; sprinkle with salt and pepper.
Place turkey atop onions in pan. Add neck, heart and gizzard pieces to pan. Roast turkey 30 minutes. Pour 1 cup broth into pan. Tent turkey loosely with foil. Reduce oven temperature to 325°F Roast turkey 2 hours. Uncover; continue to roast until turkey is golden brown and thermometer inserted into thickest part of thigh registers 180°F, basting occasionally with pan juices, about 1 hour longer. Transfer to platter. Tent loosely with foil; let stand 30 minutes.
Remove neck and giblet pieces from pan; discard. Pour onion mixture into large glass measuring cup; spoon off fat, reserving 2 tablespoons. Add more chicken broth to onion mixture if necessary to measure 5 cups total.
Heat reserved 2 tablespoons turkey fat in large saucepan over medium-high heat. Add 1 1/2 teaspoons chopped sage; stir 30 seconds. Add flour; whisk until beginning to color, about 3 minutes (mixture will be dry and crumbly). Gradually whisk in onion mixture, wine and nutmeg. Simmer until gravy thickens, whisking 5 minutes; season with salt and pepper.
Carve turkey; serve hot with gravy.
Makes 10 to 12 servings.