Put the chicken, breadcrumbs and grated cheeses into a bowl. Add the garlic, nutmeg, lemon and thyme, and the chilli flakes if desired. Using your hands, mix everything together and season well.
Wet your hands and form the mixture into meatballs, each one about the size of a walnut (in its shell). Set on a tray, cover with cling film and chill for about 45 minutes.
Heat 1 tbsp of the olive oil and brown the meatballs over a medium heat in batches. You only need to get a golden colour all over, you don’t need to cook the meatballs through. Use more oil as you need it.
In a large saucepan heat the stock to simmering and poach the meatballs in it for five minutes. Lift out and set aside, keeping the stock in the pan.
In another large pan heat the rest of the oil and sauté the onion for about 10 minutes, until soft. Add garlic and cook for another couple of minutes, then stir in the beans, turning them over in the oil. Season and leave them to suck up the flavours round them.
Tear the leafy bit from the cavolo nero – use the rib only if it’s not coarse – and chop roughly. Heat the stock again, adding the Parmesan rind if you have one. Once it’s boiling, drop in the cavolo nero and cook for about 7 minutes.
Add the stock with the cavolo nero to the pan with the beans, along with the meatballs. Add water if you need to, up to 200ml, depending on how thick you want the soup to be.
Check for seasoning – the Parmesan rind will have made it quite salty but you’ll need pepper. Remove the rind, stir in the parsley and serve with freshly grated Parmesan.