In a Dutch oven or soup pot, cook salt pork slowly until fat has been rendered and pork bits are well browned.
Remove browned bits and drain on paper towels.
Remove and discard all but enough fat to lightly coat bottom of pan (1 to 2 tablespoons).
To fat remaining in pan, add onions and cook slowly for 15 minutes.
Add potatoes and cook gently 5 minutes more, tossing once or twice.
Pour in fish stock, cover, and simmer gently until potatoes are just tender when pierced (15 to 20 minutes).
Add fish and reserved pork bits to pot and simmer, partially covered, just until fish is cooked through (about 5 minutes).
Stir in milk or cream; gently heat to a simmer without boiling.
Taste; add salt and pepper, if desired.
Ladle into heated bowls.
If desired, top each serving with dab of softened butter.