First, soak the dried cannellini beans in cold water for at least 12 hours (ideally 24 hours).
Once the beans have soaked, drain them and add them to a pot with 3 peeled garlic cloves, two sprigs of rosemary and 5 sage leaves. Cover the beans with 4 cups/1 litre water and place on a low heat, covered. *see notes if using pre-cooked cannellini beans.
Let the beans simmer gently for 2 hours then remove the beans with a slotted spoon and place in a separate bowl and discard the rosemary and sage. Keep the bean cooking liquid, this will act as your stock for the soup.
Finely chop the carrot, onion and celery and add to a large pot with the olive oil. Saute gently until soft then add the potato (peeled and cubed) and the leek, continue to cook until the leek has softened.
Add the bean cooking liquid, vegetable stock and canned chopped tomatoes along with a good pinch of salt and pepper, simmer gently for 40 minutes.
Now add the roughly chopped cavolo nero and cannellini beans, simmer covered for 20 minutes then add the chopped bread. Simmer gently for another 10 minutes then serve.