Put on a saucepan of salted water and bring to a boil. Wash the chard and separate the white stems from the green leaves. Then cut the stems into strips and cook for a good 5 minutes. In the meantime, also cut the leaves into strips and add them to the stems in the pot and let them boil once. Then strain.
Peel and slice the potatoes. Peel the onion and cut into fine strips and mix with the chard and potatoes in a bowl.
Prepare a marinade from vinegar, oil, salt, pepper and a pinch of sugar and use it to dress the salad.
The potato salad tastes best lukewarm. If you like it a little more substantial, you can e.g., give some sheep cheese over it.