Preparation Place onion slices in a medium bowl. Pour vinegar and 1/2 cup warm water over onion. Stir in sugar and season with salt; let stand until slightly pickled, about 30 minutes. Drain. DO AHEAD: Can be made 2 days ahead. Let cool slightly. Cover and refrigerate.
Preparation Place onion slices in a medium bowl. Pour vinegar and 1/2 cup warm water over onion. Stir in sugar and season with salt; let stand until slightly pickled, about 30 minutes. Drain. DO AHEAD: Can be made 2 days ahead. Let cool slightly. Cover and refrigerate.