In small bowl, combine all dressing ingredients; blend well. Place onion slices in 8-inch square (2-quart) baking dish; pour dressing over onion. Cover; refrigerate at least 2 hours or until serving time.
Remove onion from dressing. Arrange onion, tomatoes and cheese on serving platter or 6 individual salad plates. Spoon dressing over salad. Sprinkle with parsley.