1. Fry salt pork in frying pan until golden, then add onion. Continue frying until softened and translucent.
2. Transfer onions to soup pot and add potatoes and 3 quarts of water. Bring liquid to a fast simmer and cook until potatoes are just tender, about 20 minutes.
3. Add fish and continue cooking until fish falls apart into flakes and potatoes are soft. Add evaporated milk, heat thoroughly, then add salt and pepper to taste. Add butter and serve as soon as butter has melted.