Place an oven rack low in the oven; preheat oven to 375 degrees F.
Remove the giblets and neck from turkey cavities, checking that both cavities are empty. Rinse turkey with cold water, inside and out, and pat dry. Keep the legs tied together with the metal clip, or tie with butcher's twine.
Place half of the rosemary sprigs and all the onion and garlic in the bird's body cavity.
Pull the leaves off the remaining rosemary sprigs. Carefully loosen the turkey skin over the breast and legs (by running your hands under it), being careful not to tear it. Keeping the leaves as flat as possible, tuck the sage leaves and rosemary sprigs under the breast, thigh and leg skin, arranging the herbs decoratively. Carefully pull the breast skin tightly down over the breast bone, then thread metal closure skewers through both sides of the skin flaps to bridge the cavity. Thread skewers alternately from left to right, then right to left. With a 12-inch piece of butcher's twine or white cotton string, work back and forth around the skewers to lace up the cavity, shoelace-style.
Spray a roasting rack with nonstick vegetable spray and place the turkey on the rack in an open, shallow roasting pan. Rub the turkey all over with the olive oil, then generously sprinkle with the salt and pepper.
Place the turkey in the oven and roast until the inner, thickest part of the thigh registers 175 degrees F. (Insert metal stem, instant-read thermometer in the back side of the thigh by the turkey body.)
If you are cooking a larger turkey, you may need to tent the breast loosely with a piece of buttered foil to avoid over-browning of the breast. About 30 to 45 minutes before the end of cooking, remove tent to allow browning.
When the turkey is done, remove from the oven. Before carving the turkey, let it stand for 10 to 15 minutes to allow the juices to settle. As the turkey stands, the thigh temperature will reach 180 degrees F.
Use the pan drippings to make your gravy.