Preheat the oven to 190°C.
Boil 5cm potato chunks until soft, then mash with butter, crème fraîche, and warm milk.
Poach fish in milk with onion halves, bay leaf, and cloves for 7 minutes.
Fry chopped leeks and remaining onion in butter and oil until soft.
Remove the fish from the pan to cool and reserve the poaching milk.
Stir flour into the leeks and gradually add the reserved milk to thicken the sauce.
Season the sauce with salt and pepper to taste.
Flake the fish into chunks, remove skin or bones, and fold into the sauce.
Mix raw prawns and frozen peas into the fish mixture.
Place a pie dish on a baking tray.
Spoon the mixture into the dish and top with parsley and lemon zest.
Spread mashed potato over the top, dot with butter, and bake for 30 minutes until golden.