Soak the onion rings in a small bowl of ice water for about 10 minutes.
Whisk the dressing ingredients together until blended; season with salt and pepper to taste.
Drain the onion from the ice water and pat dry with paper towels.
In a large bowl, combine all the salad ingredients (except the Feta cheese).
Toss the salad with the dressing.
Sprinkle cheese over the salad and serve.