Boil the potatoes as jacket potatoes for about 25 minutes and 6 eggs hard.
Let the potatoes cool, peel and slice them in a large saucepan or bowl.
Dice the onions and pickles and pour over them. Salt and pepper.
Mix the mayonnaise:
Put the mustard in a small bowl. Add 6 egg yolks. Carefully pour in the sunflower oil as a thin stream and stir well with the whisk until a creamy mixture is formed. Pour this over the potato salad. Mix well.
Cut the bismarck herring into small pieces and slice the boiled eggs, pour over the salad and mix well again. Possibly. salt and pepper again.
Tastes best when the salad can sit through for a day.