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.)