Soak a chunk of Serrano ham in water. Soak 2 cups of white beans overnight as well. Next day fill a casserole with water. Add the drained beans, ham and slices of horizo sausage. Season well (garlic and tomatoes are often used locally). Cover with water and simmer for 2 hours, skimming when needed. Towards the end add a sliced morcilla (black pudding). If the stock is too thin, you can puree a few of the beans and add them back to the pot to thicken.