Halve, peel and finely chop the onion. Halve the tomatoes. Peel and finely chop the garlic. Thinly slice the chilli, flicking out the seeds if you want less heat.
Set a large pan over high heat. Add 1 tbsp oil and the onion, tomatoes, garlic and half the chilli with a pinch of salt. Cook over medium-high heat for 10 mins or till the tomatoes are tender and collapsing. Season with a little more salt and a big pinch of pepper.
While the veg fry, finely chop the cavolo nero. Drain the beans and rinse well. Add the cavolo nero, beans, 1 tsp of the fennel seeds and 750ml stock (homemade or made by whisking 750ml boiling water with your stock cube). Simmer for 10 mins. Grate in the lemon zest and add a good squeeze of juice. Simmer for 5 mins.
While the soup simmers, fry the remaining chilli slices and fennel seeds in 1 tbsp oil, for 2-3 mins till just fragrant. Taste the soup and adjust the seasoning. Ladle into bowls and top with the fried chilli and fennel seeds.