Remove the stalks from the cavolo nero and finely chop, tip into a wide saucepan or small casserole dish along with the oil and put over a medium-high heat. Shred the leaves and set aside. Tip the onion and rosemary into the pan with the stalks and cook for 10-12 mins until everything is slightly softened.

Stir in the garlic and sizzle for a few minutes until fragrant before pouring in the can of tomatoes. Swill out the can with the stock and pour this in as well. Bring to a simmer and let it bubble away for 5-7 mins until reduced. Stir the beans in, then nestle the fish and cavolo nero leaves into the stew. Cover with a lid and cook for a further 7-9 mins, until the fish is cooked through and tender. Season to taste and serve with crusty bread.