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, Carrots, 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.