Take the turkey out of the bag. Rinse it in cold water. Pat it dry with paper towels.
Heat the oven to 350°.
Pour out the brine from the inner bag and discard. Save the outer bag.
Mix the sage and garlic with the softened butter. Rub the herb butter over the turkey, including under the skin over the breast.
Place the apple, onions, and celery in the cavity of the turkey.
Place the turkey into the outer bag you saved from brining and place the whole thing in a roasting pan.
Roast until a thermometer registers 165° F in the meatiest part of the thigh. Start checking the temperature after 1.5 hours.
Remove the turkey from the oven, cut open and remove the baking bag, cover loosely with foil, and allow it to sit for 20 minutes. Slice and serve.