Heat oil in a large saucepan over moderate heat. Add turkey neck and giblets; brown, turning, until golden brown, about 5 minutes. Add carrots and celery; cook, stirring occasionally, until golden, about 5 minutes. Add broth and 2 cups water. Simmer until liquid is reduced to 5 cups, about 25 minutes. Strain stock through a sieve into a bowl and reserve (discard solids).
Preheat oven to 425 degree F. Rinse turkey inside and out and pat dry with paper towels. Season turkey inside and out with salt and pepper. Loosen skin from meat with fingers and carefully place sage leaves between skin and meat, keeping leaves flat. Loosely fill neck cavity with sage sprigs and some onion. Fold neck skin under turkey and secure with toothpicks. Tuck wings under and fill body cavity with more onion and sage. Tie legs together with kitchen string. Place turkey, breast side up, on a roasting rack in a large metal roasting pan. Add 1 cup water to pan.
Roast turkey in middle of oven, about 30 minutes.
Reduce oven temperature to 350 degree F and pour melted butter over turkey; continue to roast 2 hours, adding additional water (1/2 cup at a time) if pan is dry, placing remaining quartered onions around turkey about 1-1/2 hours. Roast until onions are tender but not falling apart, about 30 minutes. Transfer onions, as cooked, with a slotted spoon to a bowl; cover and keep warm. Total roasting time for turkey should be 2-1/2 to 3 hours (depending on size of bird) or until an instant-read thermometer inserted 2 inches into inner thigh (without touching bone) registers 180 degree F.
Transfer turkey to a platter, reserving roasting pan with juices for gravy, and remove toothpicks. Let turkey stand, loosely covered with foil, 30 minutes before carving.
Skim some fat from roasting pan and place pan over 2 burners. Add wine to pan to deglaze, boiling over moderately high heat until wine is reduced by half, about 5 minutes. Sprinkle flour over wine and cook, whisking, 2 minutes. Whisk in 3 cups reserved turkey stock and simmer, whisking constantly, until reduced to about 3 cups, about 7 minutes.
Strain gravy through a sieve into a medium saucepan (discard solids and keep warm, covered, over very low heat. Salt and pepper to taste.
Carve turkey and serve with onions and gravy. Makes 8 servings.