Halve, peel and finely chop the onion. Halve your tomatoes. Peel and finely chop your garlic. Thinly slice your chilli.
Set a large pot over high heat. Add 1 tbsp oil. Swirl in your onion, tomatoes, garlic and half your chilli with a pinch of salt. Cook over medium-high heat 10 mins or till the tomatoes are tender and collapsing. Season well.
Finely chop the cavelo nero. Drain your beans and rinse well. Add the cavelo nero, beans, fennel seeds and 750ml stock (homemade or made by whisking 750ml boiling water with your stock cube). Simmer 10 mins. Grate in the lemon zest and add a good squeeze of juice.
Adjust seasoning to taste. Fry the remaining chilli slices and a pinch of fennel seeds in 1 tbsp oil, till just fragrant, to spoon over the top before serving.