Cut arteries, veins, and any hard parts from beef heart; wash and set aside to drain.
Heat 1/4 cup butter in a skillet; add 1/4 cup chopped onion and cook until soft.
Turn contents of skillet into a large bowl containing a mixture of the bread crumbs, parsley, poultry seasoning, 1/4 teaspoon salt, and few grains pepper; mix lightly.
Stuff heart with the mixture and fasten with skewers; set aside.
Heat fat in a Dutch oven.
Add 1 cup onion and cook until onion is soft; remove onion and set aside.
Put beef heart into the Dutch oven; brown lightly on all sides.
Add water, beef broth, salt, pepper, celery salt, and cooked onion to Dutch oven.
Bring liquid rapidly to boiling; reduce heat, cover and simmer 2 1/2 to 3 hours, or until heart is tender.
Mix in marjoram 15 minutes before end of cooking.
Heat jelly and water together over very low heat until jelly is melted, stirring occasionally.
Remove heart from liquid.
Strain and reserve cooking liquid for gravy.
Brush beef heart with the jelly to glaze; serve with Gravy.