In a large, heavy saucepan, bring the beans and 4 cups of water to a boil. Boil briskly for 2 minutes, then remove from the heat and let stand, covered, for 1 hour.
In a 5-quart Dutch oven, melt the butter. Add the onions, celery, carrot, and garlic, and cook until soft but not browned. Add the ham hock, the remaining 4 cups water, parsley, salt, cayenne pepper, cloves, mustard seed, bay leaf, and beans (with their liquid). Bring to a boil, cover, reduce heat, and simmer for about 3 hours.
Remove the ham hocks and let cool slightly. Remove and discard bay leaf. Place about half of the beans with about 1 cup of the liquid from the soup in a blender or food processor. Process until smooth. Return the puree to the soup in the Dutch oven.
Remove the ham from its bone and add the meat to the soup. (Discard fat, bones and skin.) Reheat the soup over medium heat. Stir in the sherry. Serve hot soup with choice of garnishes to sprinkle on top.
Recipe courtesy of Brother Rick Curry, S.J, The Secrets of Jesuit Soupmaking, 2002