Rich Viennese Potato Soup

Ingredients : 7 cups plus 3 tablespoons beef broth OR chicken broth divided 2 cups onion sliced 2?½ cups sliced leeks 1?½ teaspoons dried marjoram 8 cups russet potatoes peeled and cubed ¾ cup heavy cream ½ teaspoon ground black pepper 1 pinch ground cardamom ½ pound fresh mushrooms sliced...