Disassemble herring on fillet and cut into portioned slices.
Onion, cut a quarter of the rings and sprinkle with sugar and add 1 tbsp. l lemon juice. Stir well and marinate for a few minutes.
Peel the apples and cut into approximately the same slices as the onions.
Mix onion, apples, remaining lemon juice, add sour cream, mustard and mix well. Let stand for 10 minutes and serve.