In a nonstick skillet, cook beef and onion over medium heat until meat is no longer pink; drain. , Transfer to a 5-qt. slow cooker. Layer with the potatoes, celery, carrot, lentils and barley. Combine the water, bouillon, salt and lemon pepper; pour over vegetables. Cover and cook on low for 6 hours or until vegetables and barley are tender. , Add the tomatoes; cook 2 hours longer.