Place roast, fat side up, on rack in shallow roasting pan. Sprinkle with salt and pepper. Insert meat thermometer so tip is in center of thickest part of pork and does not rest in fat. Do not add water. Do not cover. Roast in a 325 degrees oven for 1 1/2 hours. Heat 2 inches of water to boiling. Add onions. Cover and reheat to boiling. Cook until tender, about 15 to 20 minutes. Drain. Chop onions coarsely. Stir in the sage and 1/4 teaspoon pepper. Remove pork from pan. Pour off drip- pings; reserve 1/4 cup. Mound the onions in the center of the pan. Place pork roast on top. Roast until thermometer regis- ters 170 degrees (about 1 hour).