Melt 85g/3oz butter in a frying pan and sauté the finely chopped leeks over a low heat until soft. Smear 4 tin foil sheets large enough to loosely enclose each cutlet and leek, with the remaining butter. Place a generous layer of leek in the centre of each sheet, add a salmon cutlet, another layer of leek and top with a generous dollop of cream and 55g/2oz prawns. Sprinkle about 1 teaspoon of lemon juice over the top. Bring the edges of the tin foil together and make into a parcel so that none of the juices will run out. Place the parcels on a baking tray and cook in a preheated medium oven at 190°C/375°F/Gas 5 for 25 minutes.