1 medium onion, thinly sliced, about a cup
1/4 cup red wine vinegar or apple cider vinegar
1/2 tsp. sugar
sea salt
Place onion slices in 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. May be made 2 days ahead. Let cool slightly. Cover and refrigerate.
(From Bon Appetit, May 2011.)