Heat 4 tablespoons of the oil in the casserole. Add the onion and crushed dried chillies, cover and cook over a low heat for 10 minutes until soft but not browned. Scoop half the onion and chilli mixture into a small bowl and set to one side

Drain the soaked beans and add them to the pan with 1 litre of fresh cold water. Bring to a simmer, part-cover and cook gently for 45 minutes - 1 hour until the beans are tender. Add 1 teaspoon of salt, simmer for a further 5 minutes, then tip them into a colander set over a bowl to collect the cooking liquid. Measure this and make up to 600ml with water if necessary. If more, return it to the pan and boil until it has reduced to the required amount. Set to one side with the onions.

Heat the remaining tablespoon of olive oil in the cleaned casserole. Add the bacon lardons and fry for 3 minutes until lightly browned. Add the chorizo sausage and fry for another 1- 2 minutes until these too are lightly browned. Add the garlic and green chilli and fry gently for another minute, make sure that the garlic does not brown.

Stir in the reserved fried onion and chillies, the potatoes, bean cooking liquid, stock, ½ teaspoon of salt and some pepper. Bring to the boil, cover, and simmer for 10 minutes until the potatoes are almost soft.

Meanwhile, slice the green leaves of the cavolo nero away from either side of the thin stalks. Discard the stalks, bunch up the leaves and slice them across into 2.5cm wide strips.

Stir the cabbage into the soup, cover, and simmer for 5 minutes. Uncover, stir in the cooked beans and simmer for another 2 - 3 minutes until the cabbage leaves are tender, the beans have heated through, and the potatoes are just beginning to break apart. Season to taste with salt and pepper.

Ladle the soup into warmed bowls and serve sprinkled with some chopped flat leaf parsley.