Put the fish in a large bowl, cover with 1.2 litres boiling water and leave for 8-10 mins until it flakes easily. Remove it with a slotted spoon, peel off skin and flake into large chunks. Keep the warm liquid.
Cook the leek in butter in a large frying pan until softened. Stir in the rice, then pour over the fish poaching liquid. Give it a good stir, then let it bubble for 15-20 mins until the rice is nearly cooked.
Carefully stir in fish flakes without breaking them up. Cook rice for 2 mins more, stirring occasionally until cooked and most of the liquid has evaporated. Stir in mascarpone and sprinkle with chives if using.