I'd go with red wine vinegar, EVOO, pinch of sugar, S&P, and some cheese (parmesan or feta). Add a couple olives and some lettuce and you got the classic Greek or Italian salad. Don't forget to rinse the cut onions before you add them to any raw dish.
I don't mean to speak for him, but I'm pretty sure rinsing them in cold ice water in particular dulls the taste of onions. Especially for red onions which are very overpowering.
What does that do, NoMayo?
Ok so I need a dressing for a cucumber tomato onion salad. It's is such a cool and refreshing summer dish, but it needs the right pizazz to it.