Peel the onions and slice into very thin rings. Cover with cold water and soak for 30 minutes. Drain Stir the sugar and vinegar together until the sugar is dissolved. Pour over the drained onions. Cover tightly and refrigerate for a few hours, stirring once or twice and pushing the onions under the vinegar. Use in salads, or in any dish that uses raw onions. Prep time doesn't include soaking or marinating time.