Preheat the oven to 325°. Sprinkle turkey inside and out with salt and pepper. Tuck wings under the turkey. Place the turkey in a large oven roasting bag. Scatter onions, carrots, celery, turnip and garlic over and around the turkey. Fold the top of the bag over and place turkey, breast up, in a small roasting pan. Add water to a depth of about 1 inch. Roast turkey 3 to 3 1/2 hours, or until a meat thermometer inserted in the thickest part of the leg (but not touching the bone) reaches 165° to 170°.
Carefully tear open and remove the bag. Increase the oven temperature to 450°. Roast 30 minutes longer, or until the thermometer registers 180°, basting with the pan juices every 5 minutes. Turkey should be golden brown.
Remove the roasting pan from the oven and carefully transfer turkey to a platter, letting the juices run back into the pan; reserve the juices for gravy. Cover turkey with foil and let stand for 20 minutes before carving.