1. Place the haddock in a large pan, skin side down, cover with water, bring to the boil, and simmer 5-8 minutes until the fish is just cooked (Finnan-Haddie will take longer to cook than haddock fillets).

2. Take the fish from the pan, remove skin and bones, and discard.

3. Flake the fish and put it back in the stock with the onion. Add seasoning to taste and simmer for 30 minutes.

4. Strain and reserve the stock and fish.

5. Add milk to the fish stock. Then add enough mashed potato to make your preferred consistency.

6. Add the fish, reheat and taste for seasoning.

7. Add the butter, which will barely melt but will run through the creamy soup.

As an alternative to the smoked Finnan-Haddie you could use 1 kg (2 lbs) smoked haddock fillets.