Preheat the oven to 450F.

Rinse the bird, inside and out, under cold running water. Pat dry.

Separate the skin of the turkey from the breast and thighs by running your fingers under the skin, breaking any fibers between the two. Insert the sage leaves underneath the skin, lying them flat against the turkey meat. Lightly press the skin back in place. Rub the turkey all over with the olive oil.

Fill the cavity of the turkey with the thyme and sage sprigs, carrot, celery, onion, and garlic. Fasten the legs close to the body by tying the drumsticks together with kitchen twine. Season with salt and pepper.

Place the turkey in the oven for 25 minutes, then lower the temperature to 350F. Roast until the juices run clear between the thigh and breast when pierced with a fork, or until the internal temperature registers 150F with a meat thermometer. Cooking time will vary according to the size of the turkey, but will be about 5 hours for an 18 pound bird.

Meanwhile, in a saucepan, prepare the gravy. Brown the neck, giblets, and onion in butter. Add the carrot and celery. Add the water and sachet, and bring to a boil. Let simmer over low heat during the time the turkey is roasting, adding more water, if necessary.

When the turkey is done, remove the vegetables from the cavity and discard. Transfer the turkey to a serving platter and keep warm. Place the roasting pan over high heat until the fat separates from the brown bits. Pour off as much fat as possible, then add the wine, scraping the bottom of the pan to loosen the brown bits. Simmer for 5 minutes until reduced by half.

With a slotted spoon, lift the neck, giblets, and vegetables from the stock and discard. Add 4 cups of stock to the roast