Preheat the oven to 180˚C, gas mark 4. Put the oil in a flameproof casserole dish (or ovenproof pan) over a medium-high heat. Fry the onion and garlic with a pinch of salt for 3-4 minutes, stirring occasionally, until the onion is starting to soften. Add the rice and cook, stirring, for 2-3 minutes. Pour in the white wine and simmer for 2 minutes until mostly evaporated, then add the stock. Bring to a simmer, cover, then transfer to the oven for 35 minutes.
Stir in the cavolo nero, then nestle the fish on top. Cover and return to the oven for about 15 minutes, until the haddock is cooked through and opaque. Roughly break up the fish with a fork, then stir it through the rice. Season, then stir in the lemon zest and most of the cheese. Serve with the remaining cheese scattered over and lemon wedges for squeezing.
