In A Large Resealable Plastic Bag, Combine The Flour, Salt And Pepper. Add Beef; Seal Bag And Shake To Coat. Place Onion In A 3-Qt. Slow Cooker Coated With Nonstick Cooking Spray. Layer With The Beef, Celery And Tomato Sauce. Cover And Cook On Low For 8 Hours Or Until Meat Is Tender.