Bring to boil and simmer 3 cups water with the fish bones and trimmings together with the seasonings and a slice of onion. Meanwhile, fry the salt pork in a soup pot with the rest of the onions, chopped. Add the potatoes and fish, cubed, and cook about 10 minutes. Strain the fish stock into the soup pot. Add the milk and butter, bring to just under a boil, and then season to taste.