Boil potatoes in lightly salted water in large pot for 35 to 40 minutes or until just tender, being careful not to overcook. Drain well.
Peel cooked potatoes while still hot. Cut into slices about 1/3 inch thick. Place sliced onion in a large bowl and place hot potato slices on top. Cover with plastic wrap; set aside for at least 1 hour.
Mix together vinegar, garlic powder, salt, pepper and nutmeg in small bowl. Whisk vinegar mixture into mayonnaise until smooth.
Add chopped egg to potato mixture and gently stir to combine. Gently fold in mayonnaise dressing, olives and parsley. Cover with plastic wrap. Refrigerate for at least 3 hours, preferably overnight, before serving. This may be done up to 2 days ahead, covered and refrigerated.