Cut the fish into large, bite-sized pieces and put in a bowl with the prawns and peas. Add the lemon rind and juice, stir in the white sauce and dill and season well with salt and pepper.
Tip the fish mixture into 4 individual gratin or pie dishes. Cover the surface of each pie with 2 sheets of filo pastry, scrunching up each sheet into a loosely crumpled ball. Brush the pastry with melted butter.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–25 minutes until the pastry is golden brown and the fish is cooked through.