In a kettle combine the potatoes with enough cold water to cover them by 1 inch and simmer them for 15 to 20 minutes, or until they are just tender. Drain the potatoes and let them cool. The potatoes may be prepared up to this point 1 day in advance and kept covered and chilled. Cut the potatoes into 1-inch pieces and in a large bowl toss them with the onion, 2 tablespoons of the oil, the rosemary, and salt and pepper to taste. In a large non-stick skillet heat 3 tablespoons of the remaining oil over moderately high heat until it is hot but not smoking and in it saute half the potato mixture, stirring, for 10 to 15 minutes, or until the potatoes are golden. Transfer the potatoes to a serving dish and keep them warm, covered. Cook the remaining potato mixture in the remaining 3 tablespoons oil in the same manner, transfer it to the serving dish, and toss the potatoes with the parsley and salt and pepper to taste.