Disassemble herring on fillet and cut into pieces.
For the sauce, mix mustard, mayonnaise, pepper, sugar 1 tsp., Lemon juice and mix. Add the herring to the sauce and mix.
Cut the onions in half rings, sprinkle with vinegar and add the remaining sugar, mix well and put on a serving dish.
Place the herring in the sauce on the onion beautifully and refrigerate for at least half an hour, preferably an hour. Sprinkle with chopped herbs before serving.