Cut the onion into half rings.
Mix the vinegar, oil, and mustard into a homogeneous emulsion.
Add a little salt to the onion, mash it with your hands and pour the dressing over it.
Wash the potatoes thoroughly, put them in a saucepan, cover with water and cook until tender for 20-25 minutes.
Peel the boiled potatoes and cut them into half-circles.