Marinating the onion and cucumber in the vinaigrette tones down the onion's harshness and flavors the cucumber. For efficiency, prepare the other salad ingredients while the onion and cucumber marinate. Use a salad spinner to dry the lettuce thoroughly after washing; any water left clinging to the leaves will dilute the dressing.
Whisk vinaigrette ingredients in large bowl until combined. Add onion and cucumber and toss; let stand to blend flavors, about 20 minutes.
Add romaine, tomatoes, parsley, mint, and peppers to bowl with onions and cucumbers; toss to coat with dressing.
Transfer salad to wide, shallow serving bowl or platter; sprinkle olives and feta over salad. Serve immediately.
