Place the haddock fillet in a large frying pan and cover with boiling water.

Let it stand for 10 minutes, then drain, reserving 300ml/10fl oz of the soaking water.

Flake the fish, ensuring to remove all the bones.

Melt the butter in a large saucepan over low heat.

Add the onion and sweat gently for 10 minutes until softened.

Pour in the milk and bring to a gentle simmer before adding the diced potatoes.

Cook for 10 minutes.

Add the reserved haddock flakes and the cubed cod.

Simmer for a further 10 minutes until the cod is tender.

Remove about one-third of the fish and potatoes, transfer to a blender or food processor, and purée until smooth.

Return the mixture to the soup along with the cream, parsley, and salt and pepper to taste.

If the soup seems too thick, add a little of the reserved soaking water.

Reheat gently and serve hot.