In a bowl, combine cornstarch, broth, soy sauce and curry powder; stir until smooth. set aside. In a large skillet or wok, saute lamb, onion and garlic in 1 tablespoon oil until meat is browned. Remove and keep warm. In the same skillet, stir-fry apple, green pepper, celery, water chestnuts, peas and ginger in remaining oil until crisp-tender. Add lamb mixture. Stir broth mixture and add to skillet. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve over rice.