Soak overnight in salted water to draw out the blood is what I've read.Here's 2 recipes
1) Soak over night as above, then stuff with
2cups bread cubes,1 stalk dicedcelery,1 tbs chopped onion,1/4tsp marjoram,1 tbs melted butter and milk to moisten. Stuff heart and bake 350degrees 1 1/2-2 hrs Baste with juices or water. Thats how the recipe read but I would tent it with foil.
2)Pickled Heart- 3/4 vinegar to 1/4 water or to desired strenth, add some pickeling spices and sliced onion, Cut heart in half add onion,celery,and a bay leaf, simmer about 30 min or till tender,cool and slice, salt and pepper to taste and cover with pickleing liquid. refrigerate overnight then just eat at will.