At the same time, you can prepare salad dressing:
peel the onion, finely chop, pour in the broth, bring to a boil and cook for 5 minutes.
Then add mustard, vinegar, salt to taste and more black pepper.
Beat the puree with a blender. Add vegetable oil and mix.
Immerse the prepared potatoes in the hot dressing and leave for an hour. Periodically, during this time, gently stir the potatoes.