Skin and bone cod fillet and haddock and cut both into large dices (2.5cm / 1 in) and set aside. Prepare 600ml (1 pt) fish stock from the fish stock cube and set aside.
Melt butter in a large saucepan. Cook and stir the onion until soft. Add milk, fish stock and potatoes - simmer until potatoes are soft. When potatoes are cooked, add the cod and haddock. Poach gently - do not boil. Add cream and prawns and heat until prawns and fish are cooked through and opaque.