Put 2 litres/3½ pt of water in a heavy saucepan which has a well fitting lid, add the borlotti beans, ham hock, onion, garlic, celery, rosemary (wrapped in muslin), parsley and olive oil. Bring to the boil, lower the heat and gently cook the beans for 1½ hours. Discard the ham hock, and rosemary, remove the vegetables and blend them. Put half of the beans through a food mill or liquidizer and return the purée to the soup of beans and vegetables. Do not replace the lid. Season with freshly ground black pepper and check the seasoning: salt may not be necessary. Bring back to the boil. Add the pasta and cook until al dente. Serve with a dash of extra-virgin olive oil and a pinch of chilli pepper to taste.