Scatter the pancetta into a deep flameproof casserole dish and cook on a medium heat for 5-6 mins until crispy and releasing its fat. Remove from the pan using a slotted spoon, leaving any fat behind. Pour in the oil, then tip in the onion, celery and carrot. Cook for 12-15 mins until turning golden. Stir in the garlic and cook for 1 min more.

Pour in the beans, chicken stock and parmesan rind (if using). Cook for 15 mins on a medium heat, then stir in the cavolo nero and cooked pancetta for a final 5 mins. Stir in the sherry vinegar and season.

Divide between four bowls and sprinkle with chives to serve.