The finished heart will still be slightly chewy, but not unpleasantly so. Apply salt to taste. Add onion, celery, bread cubes and enough beef broth to moisten. For stuffing, cook onion in shortening until tender. Slice heart in half and remove the hard parts. Prepare heart for stuffing. Simmer the heart for 4 to 5 hours in your slow cooker on Low, or half that time on High. 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.