Remove the giblets and neck; reserve the neck. Rinse out the turkey’s cavity and thoroughly pat dry with paper towels. Trim most of the excess fat and skin from the neck and cavity, and make 3-inch slits through the skin where the legs meet the breast.
Rub the turkey all over with 2 tablespoons of the vegetable oil, then season generously with salt and freshly ground black pepper. Season the cavity with salt and pepper, and place 1 onion half, 1 celery piece, and 2 garlic cloves inside.
Place the turkey in a large roasting pan. Arrange the neck and remaining onions, celery pieces, and garlic cloves in the pan, and place in the oven. Roast for 30 minutes, then lower the temperature to 350°F. Every 45 minutes, baste the bird with the pan drippings.
About 45 minutes before the turkey is finished or when the internal temperature of the inner thigh reaches 145°F, cut the pears in half and remove the cores and stems. Brush each half with the remaining 1 tablespoon vegetable oil and season well with salt and freshly ground black pepper. Remove the turkey from the oven and overlap bacon strips across the breast and around the legs. If desired, secure the bacon strips about 1 inch from the edges with toothpicks. Arrange the pear halves in the roasting pan and return the turkey to the oven.
Roast until the internal temperature of the inner thigh reaches 155°F. Remove the turkey from the oven and let it rest uncovered while you prepare the gravy, or for at least 30 minutes before carving. Remove the pears to a serving platter, reserve the onions, and discard any remaining solids in the roasting pan.