Preheat oven (200 °C, Fan 180 °C, 400 °F, Gas 6).
Spread out the sweet potato chunks on trays and bake in the oven for 60 minutes - or until they can be easily sliced in half. While they’re still warm from baking, mash them in a large bowl until they’re starting to break down and form a sticky mixture (alternatively, use a food processor).
Heat a large, non-stick frying pan. Add the oil and onion, stirring and cooking for a couple of minutes until translucent. Add this to the mashed vegetables.Put the cavolo nero and a few tablespoons of water into the hot frying pan and cook for a couple of minutes until softened, then add it to the mashed vegetables.Stir thoroughly to distribute the additions. Season the mixture generously with salt and pepper. Once cooled enough to handle, use your hands to squeeze and press everything together, mixing it up and forming a sticky mass. Divide into eight balls and press these flat to form fritters.
Fry the fritters in batches in a preheated, well greased pan until golden, flipping over half way to cook the other side. A low heat and long slow cooking brings out the best flavour and crisp crust, but keep a close eye on these as they can brown fast. Serve hot and fresh, garnished with some chives and brown sauce.