Preheat oven to 450°. Combine salt and pepper in a small bowl. Using a sharp knife, cut a line down into the roast about 1/2 in. in front of the ribs to create a deep pocket into which you'll stuff the rosemary and garlic. Sprinkle the pocket with about half the salt mixture, then insert rosemary and garlic, allowing rosemary sprigs to poke out of the roast. Tie the roast between the ribs with kitchen string. Sprinkle the remaining salt mixture on the outside of the roast. Place in a large roasting pan with ribs pointing up.
Cook 15 minutes, then reduce oven temperature to 350° and roast until a thermometer inserted in center of meat reaches 130° for rare, 75 to 90 minutes, or 140° for medium, about 2 hours. Remove from oven, tent with foil, and let rest for 30 minutes. Cut strings and remove rosemary. Carve and serve.
Note: Nutritional analysis is per serving.