Boil the potatoes in their skins in plenty of salted water until just tender. Run cold water over them, drain, and leave to cool enough to handle. To pickle the onions, place them in a saucepan just large enough to hold them, pour in the vinegar, and 2 tablespoons water, and sprinkle with sugar. Cover the pan and bring to a boil. Stir, cover, and simmer for 1 minute. Shake the covered saucepan and put it aside for 5 minutes to steam. Peel and slice or roughly dice the potatoes and place in a serving bowl. Sprinkle with the olives, capers, and pickled onions with their juice olive oil, garlic and parsley. Toss and season to taste.