At certain times of the year it's impossible to find sweet onions. The strong onions are really way too harsh in a gentle salad. In a restaurant we had this on our salad, and it wasn't hard to figure out how to imitate it. It's so very easy ... and you can, of course, also serve this on meats, chicken, cheese and with egg dishes!! For the best flavour marinate for at least 48 hours! Quantities are really of no concern, as long as your vinaigrette covers the onion rings.