If using dried borlotti beans, soak them overnight in plenty of cold water. The next day, drain and rinse well.
If using tinned beans, drain and rinse one tin of beans. Use the second tin with its liquid, as this helps give the soup a lovely creamy texture.
Warm the olive oil in a large heavy-based saucepan over low–medium heat. Add the onion, celery, carrot and fennel and cook gently for around 12–15 minutes, stirring often, until soft and aromatic but not browned.
Add the garlic, rosemary, bay leaf and sage, if using, and cook for another minute, stirring.
Add the beans and stir to coat them well in the vegetables and oil. Pour in the stock and add the parmigiano rind, if using. Bring to a gentle simmer, then reduce the heat and cook for around 20 minutes if using tinned beans.
If using dried beans that have been soaked overnight, simmer gently for around 1–1½ hours, or until the beans are tender. Add salt once the beans have started to soften.
When the beans are tender, remove the bay leaf, rosemary stalk and parmigiano rind. This next step is optional but it does give the soup a beautiful texture. Take out a few ladlefuls of the soup and blend until smooth, or simply crush some of the beans against the side of the pot with the back of a spoon. Stir this back through the soup. This gives pasta e fagioli that beautiful creamy texture without adding cream.
Add the pasta and cook until al dente. Small pasta usually takes around 6–8 minutes. Stir often so the pasta does not catch on the bottom of the pot. Add a little extra stock or water if the soup becomes too thick. It should be hearty and spoonable, not dry.
Stir through the Cavalo Nero.
Taste and adjust the seasoning. Ladle into bowls and finish with a generous drizzle of your best extra virgin olive oil, a little black pepper, fennel fronds if you have them, and grated parmigiano if you like.