Cut the swordfish into large chunks, discarding any skin and bones, and scatter in a 1.5 litre (2½ pint) ovenproof dish or pie dish. Add the prawns and season with salt and pepper.
Beat the mascarpone in a bowl to soften. Stir in the wine and spoon over the fish.
Chop the lemon grass as finely as possible and process in a blender or food processor with the flour and butter until the mixture resembles fine breadcrumbs. Add the dill and pulse very briefly to mix.
Tip the mixture over the fish and sprinkle with the Parmesan. Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for about 35–40 minutes until the topping is pale golden. Serve with a green salad.