Add onion, celery, bread cubes and enough beef broth to moisten. Prepare heart for stuffing. Apply salt to taste. Simmer the heart for 4 to 5 hours in your slow cooker on Low, or half that time on High. Slice heart in half and remove the hard parts. For stuffing, cook onion in shortening until tender. If you're braising the heart in a Dutch oven or baking dish, place it an an oven preheated to 325 F for 3 to 4 hours. Close the slit with metal skewers, then lace and tie with string forming pocket. The finished heart will still be slightly chewy, but not unpleasantly so.