Cook the cavolo nero in a large saucepan of boiling water for 3 minutes, drain. Reserve the saucepan.
Meanwhile, heat half the oil in a large frying pan and fry the onion for 4-5 minutes until golden, tip half into the saucepan and set aside.
Add the spices to the onion in the frying pan and cook for 1 minute. Transfer to the bowl of a food processor with the chick peas, 50g of cooked cavolo nero and the flour. Season and blitz to give a coarse paste. Divide into 12 balls with damp hands and flatten slightly.
Add the chopped tomatoes and stock to the saucepan, bring to the boil and simmer for 10 minutes until thickened. Stir in the remaining cavolo nero and cook for a further 3 minutes, season.
Heat the remaining oil in the empty frying pan and fry the falafels for 3 minutes each side or until golden.
Serve the falafels with the tomato sauce and couscous.