Preheat oven to 325 degrees.
Rinse turkey with water and pat dry. Remove giblet packet from cavity
Stuff cavity with onion quarters (and extra rosemary sprigs if desired).
Place sheet of aluminum foil into roasting pan, leaving enough on each side to wrap the turkey.
Blend butter, rosemary and garlic. Using your fingers, spread mixture on the entire surface of the turkey. Sprinkle with salt and pepper.
Wrap turkey loosely with foil (not airtight) and place in oven.
Roast in oven 3-4 hours.
Roast 3-4 hours. Unwrap foil for the last 30-45 minutes of cooking time to crisp the skin.
Remove turkey; discard onion and rosemary. Transfer turkey to carving plate and let rest a few minutes before slicing.