Soak the herring in water to cover overnight. Change the water twice. Drain.
Chop the onion, apple, eggs and herring together. Pour the vinegar over the bread and add to the herring with the sugar and oil. Chop until very smooth. Taste for seasoning, adding more vinegar if needed. Chill.
Serves 8 as an appetizer or as many as 24 as a spread.