Put a large pot of water on the stove to boil. Place a large pan over a medium heat and heat half of the oil. Add the onion to the pan, stirring while cooking. When the onion begins to caramelise, add the garlic, chilli, tomatoes and cavolo nero. Cook until the tomatoes have gone soft, stirring regularly. Turn down to a low heat and add the crab, juice of the lemon, finely grated zest of the lemon and the remainder of the olive oil.