Poach the fish – Simmer onion, cloves, and bay leaf with 900ml milk, cream, pollock, and haddock for 6 minutes. Remove fish, strain liquid. Flake into large pieces and place in an ovenproof dish.
Cook the eggs – Hard-boil for 8 minutes, cool, peel, and slice. Arrange over the fish.
Make the sauce – Melt 100g butter, stir in flour, then gradually whisk in the poaching liquid. Simmer for 10 minutes. Add parsley, prawns, nutmeg, salt, and pepper. Pour over fish, cool, and chill for 1 hour.
Prepare the mash – Boil potatoes for 15–20 minutes, drain, and mash until fluffy. Beat in butter, egg yolks, seasoning, and enough milk to loosen.
Bake – Preheat oven to 200ºC/180ºC fan. Spread mash over filling, mark with a fork, and bake for 35–40 minutes until golden brown.