In medium bowl whisk all the ingredients except the oil until mixed. Gradually whisk in the oil until smooth.
In a small bowl, cover the onion with iced water and let stand for 10 minutes. Drain well. (Soaking the onion removes the sharp bit).
In a large bowl, place the lettuce, drained onions, tomato and cucumber and add the dressing a little at a time, tossing gently and thoroughly until all the vegetables are evenly coated. Season with additional salt and pepper to taste.