Cook onion, celery, and garlic in hot olive oil in an 8- or 10-quart Dutch oven or kettle until tender. Remove from Dutch oven; set aside.

Trim excess fat from lamb shanks. Add half of the shanks to the Dutch oven, and brown on all sides; remove. Repeat with remaining shanks. Carefully drain fat from Dutch oven. Return all shanks and cooked vegetables to the Dutch oven. Carefully add chicken broth, wine or reduced-sodium chicken broth, the four sprigs fresh rosemary, salt, and pepper. Bring to boiling; reduce heat. Cover and simmer for about 2 hours or until lamb is just tender.

Stir lentils into Dutch oven. Simmer, covered, for 20 minutes. Add tomatoes, sweet potatoes or yams, and orange peel. Simmer, uncovered, 15 to 20 minutes more or until lentils and sweet potatoes are tender. Remove rosemary stems from pot; discard. Skim fat from broth.

To serve, place a lamb shank in each of eight shallow pasta or soup bowls. Ladle ragout mixture into each. If desired, garnish with fresh rosemary sprigs. Makes 8 servings.

Cool cooked lamb and ragout mixture for 1 hour. Divide mixture between two large refrigerator containers or 3-quart casseroles. Cover and chill in the refrigerator overnight. Remove any excess fat. Return to large Dutch oven and reheat over medium-low heat, stirring constantly.