Rinse the beans in hot water until they are white. Place them in a stockpot, add the water, and bring to a boil. Reduce the heat and simmer for 3 hours. Heat the butter in a small skillet over medium heat. Saute the onion until lightly browned. Add the onion to the pot of beans. Remove 2 cups of the bean mixture, puree in a blender, and return to the soup. Add to ham to soup mixture. Just before serving, season the soup with salt and pepper, to taste. Garnish with parsley.
*Cook's note: This recipe can also be made with ham hock.