Cover the fish fillet in a large saucepan with plenty of cold water and bring to the boil. Add the chopped onion and cook for 10-15 minutes. Remove the fish and loosen the meat from the bones. Return the bones and skin to the broth and cook for another 30 minutes.
Carefully sift the broth into a clean saucepan. Add the fish meat, add the milk, salt to taste and cook for a few minutes.
Mashed potatoes (amount to taste - the mashed potatoes act as a binding agent for the soup), add butter and pepper and serve immediately.
Part of the milk can also be replaced with cream to refine it.
A hearty fisherman`s meal from the east coast of Scotland.