Heat olive oil in a large saucepan over medium heat.

Brown turkey neck and giblets in the saucepan, turning occasionally, until golden brown (about 5 minutes).

Add chopped carrots and celery to the saucepan and cook, stirring occasionally, until golden (about 5 minutes).

Pour in chicken broth and 2 cups of water into the saucepan.

Simmer the mixture until the liquid reduces to 5 cups (about 25 minutes).

Strain the stock through a sieve into a bowl and reserve the liquid.

Discard the solids.

Preheat oven to 425°F.

Rinse the turkey inside and out and pat dry with paper towels.

Season the turkey inside and out with salt and pepper.

Carefully loosen the skin from the meat with your fingers and place fresh sage leaves between the skin and the meat, keeping the leaves flat.

Loosely fill the neck cavity with sage sprigs and some quartered onion.

Fold the neck skin under the turkey and secure it with toothpicks.

Tuck the wings under the turkey and fill the body cavity with more quartered onion and sage.

Tie the legs together with kitchen string.

Place the turkey, breast side up, on a roasting rack in a large metal roasting pan.

Add 1 cup of water to the bottom of the roasting pan.

Roast the turkey in the preheated oven for about 30 minutes.

Reduce the oven temperature to 350°F and pour melted butter over the turkey.

Continue to roast for 2 hours, adding additional water (1/2 cup at a time) if the pan becomes dry.

After 1 1/2 hours, place the remaining quartered onions around the turkey in the pan.

Roast until the onions are tender but not falling apart (about 30 minutes).

Transfer the cooked onions with a slotted spoon to a bowl, cover, and keep warm.

The total roasting time for the turkey should be 2 1/2 to 3 hours, or until an instant-read thermometer inserted 2 inches into the inner thigh registers 180°F.

Transfer the turkey to a platter, reserving the roasting pan and juices for gravy, and remove the toothpicks.

Let the turkey stand, loosely covered with foil, for 30 minutes before carving.

Skim some fat from the roasting pan and place the pan over two burners on the stovetop.

Add wine to the pan to deglaze, boiling over medium-high heat until the wine is reduced by half (about 5 minutes).

Sprinkle flour over the wine and cook, whisking constantly, for 2 minutes.

Whisk in 3 cups of the reserved turkey stock and simmer, whisking constantly, until reduced to 3 cups (about 7 minutes).

Strain the gravy through a sieve into a medium saucepan and discard the solids.

Keep the gravy warm, covered, over very low heat.

Season the gravy with salt and pepper to taste.

Carve the turkey and serve with the cooked onions and gravy.