Wash the potatoes and boil in boiling water for 20-25 minutes. Peel onions and cut into cubes. Heat oil in a pot, cover and stew onions for about 15 minutes. Add mustard and broth and bring to the boil. Add vinegar and ham, season stock with salt and pepper.
Drain the potatoes, quench, peel, slice and add to the stock. Leave to stand in the broth for at least 20 minutes.
Wash the chives, cut them diagonally into fine rings and add to the salad. Season with salt, pepper and sugar. Serve potato salad with broth warm or cold.