Place the haddock in a shallow bowl. Pour over the hot stock and leave for 5 minutes until nearly cooked through. Strain the stock through a sieve into a saucepan and keep hot. Break the fish into large flakes, discarding the skin and any bones.
Meanwhile, heat the butter and oil in a large frying pan. Add the leek and a splash of water, cover and leave to gently cook for 7 minutes, stirring occasionally, until soft. Stir in the rice until well coated, then pour in the wine and cook for 2 minutes until nearly boiled away.
Add the reserved hot stock, a ladleful at a time, stirring and simmering after each addition until the stock is absorbed before adding the next. After about 15 minutes when all the stock is absorbed and the rice is nearly cooked, add the fish and cook for a further 2 minutes until the rice is tender and still very moist. Stir in the mascarpone and season with salt. Cover and leave to stand for 2 minutes.
Spoon into serving bowls and scatter with the chives.