Peel and finely chop the onion. Peel and finely chop or grate the garlic cloves (set 1 chopped clove aside in a large bowl to use later in the recipe). Roughly chop 1 tomato. Trim any woody ends off the cavolo nero and shred the leaves.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the onion, 1 chopped garlic clove and a pinch of salt and pepper. Fry for 3-4 mins, stirring every so often.
Drain the borlotti beans and rinse under a cold tap. Add to the pan along with the chopped tomato. Crumble in the stock cube and top up with 750ml boiling water. Bring back to the boil, then reduce to a simmer and cook for 12 mins.
While the soup is cooking, halve the chilli, scraping out any seeds and white membrane for less heat, and finely chop it. Finely chop the remaining tomato. Slide both into the bowl with the remaining chopped garlic clove. Grate in the lemon zest and squeeze in the juice. Sprinkle in a little salt and pepper and stir well.
When the soup has cooked for 12 mins, stir in the cavolo nero and continue to cook for a further 3-4 mins, till wilted. Use a stick blender to whizz the soup a little, but leaving it quite chunky. No stick blender? Transfer a couple of ladlefuls of soup to a food processor and blend, then pour back into the pan. Taste and add more salt and pepper if needed.
Ladle the soup into bowls and serve topped with generous spoonfuls of the salsa.