Cook the potatoes with their skin on in plenty of salted water, peel them and let them cool down well. It is best to cook the potatoes a day in advance. Cut into thin slices, you should always be able to see the knife through the potato, then the starch is correct. Put the onion cubes with the potato slices in a bowl and season everything lightly with salt and pepper.
For the mayonnaise, the ingredients should all be room temperature. Put the three egg yolks in a bowl, mix with the mustard and season with salt and pepper. Stir in the oil drop by drop, later in a stream. Add the lemon juice to the finished mayonnaise. Season to taste and season again if necessary.
Add the mayonnaise to the potatoes and fold in carefully. Let the salad soak through and then season to taste again, usually there is still a little acid missing, which can be remedied with a little vinegar. Finally fold in the chopped pickles and sprinkle the parsley on the salad.