How to Make It
In a large enameled cast-iron casserole, heat the 3 tablespoons of oil. Add the pancetta and cook over low heat, stirring occasionally, until golden, 4 minutes. Add the garlic, carrots, celery, onion, fennel, parsley, rosemary and basil and cook over moderate heat, stirring occasionally, until the vegetables are softened, 10 minutes.
Add the tomatoes, beans, potatoes and prosciutto skin to the casserole and cook, stirring, until the vegetables are just heated through, about 3 minutes. Add the chicken stock and bring to a simmer. Cook over moderately low heat until the beans are tender, about 40 minutes. Discard the prosciutto skin and the herb sprigs.
Partially mash the beans with a potato masher, or transfer 2 cups of the soup to a food processor or food mill and puree; return the puree to the casserole. Season the soup with salt and pepper. Ladle the soup into bowls and drizzle with olive oil. Garnish with the celery leaves and serve.
Any combination of fresh shell beans can be used. Cooking time will vary according to the variety of bean.