Soaking onion slices in a red wine vinaigrette tames their bite while flavoring the vinaigrette with onion.
1. Place onion, oil, vinegar, sugar, and oregano in 1-qt. resealable plastic bag. Seal, and shake to coat onion with liquid. Refrigerate 4 hours, or overnight.
2. Toss together lettuce, cucumbers, tomatoes, feta, and olives (if using) in bowl. Add onions and vinaigrette, and season with salt and pepper, if desired.