Skin the haddock. Save the head and the tail. Cut out the backbone. Save same. Cut the fish into 2-inch pieces. Put the head, tail, backbone, and any odd remnants of meat into a saucepan. Add cold water. Bring slowly to the boiling point, then let simmer for 30 minutes. Place pork bits in another saucepan to try 'em out till crisp and browned. Empty pork bits, standing them aside. Into their fat, add the onions, frying them slowly for 5-10 minutes, till browned. Remove onions. Stir in the flour. Slowly add the broth drained from the bones, stirring continuously to avoid lumping. Add diced potatoes; onions, and fish. Cover. Simmer slowly for an hour, until the potatoes lose their stiffness some. Add hot milk, butter, salt and pepper to taste. Add minced parsley. Simmer 5 minutes longer. Serve soup accompanied with warmed pilot biscuits, oysterettes, or common crackers.