Peel, wash and slice the potatoes. Peel the onions, dice finely, sauté in the butter until translucent. Pour in the broth and bring to the boil. Add the potato slices and cook covered for 10 minutes.
In the meantime, dice the mustard pickles. Cut chives into rolls, radishes into slices. Whisk the egg yolks with the spices, lemon juice and about 3 tablespoons of the mustard pickle vinegar.
Stir the egg yolk mixture into the potatoes, then stop boiling.
Finally stir in the cucumbers, radishes and chives and serve.