German Liver Dumpling Soup

10 cups beef stock 1 small onion, finely diced or 1/2 large onion, finely diced 350 grams sandwhich bread 1 egg 1 egg yolk 300 grams liver, ground salt, pepper, nutmeg to taste 1/2 tablespoon dried marjoram 1/4 cup fresh finely chopped parsley zest from one small lemon 1 bay leaf 1 tablespoon dijon mustard * may need to adjust the consistency of the dumpling using egg yolk or beef stock if too dry or more bread crumbs if too wet* ** fresh parsley for garnish when serving** *** best made the day before***