Boil potatoes - not too long. Peel while it is still hot, allow to evaporate. When they are cold, cut into not too thin slices (3-5mm thick). Steam the onions and ham in olive oil until translucent. Deglaze with water and bring to the boil with the instant stock. Pour the onion and ham mixture over the potato slices, add mayonnaise, salt and vinegar and stir carefully. Cover and season again after 2 hours, possibly adding vinegar or salt.