Step 1Soak the beans in the water in a pot overnight.
Step 2Add the ham bone to the pot and simmer the beans until they start to become tender, about 1 hour. Mix in the mashed potatoes until smooth. Add the onions, celery, garlic and parsley and simmer until the beans are soft, about 1 hour.
Step 3Remove the ham bone, then dice the meat and return it to the soup. Thin with hot water, if necessary (the soup should be thick). Season to taste with salt and pepper. Garnish with lemon slices.