This is one of my all-time favorite hors d'oeuvres to serve at get togethers and also for late nite snacks. The onions and the mayo melt together under the broiler and the taste is out of this world!.. they just couldn't be any easier to put together! If you can find Vidalia onion, then use it for this. The recipe can easily be doubled of coarse. If you are a mustard lover then add on a teaspoon of sweet or Dijon mustard on top of the onion, also if you want, sprinkle some grated Parmesan cheese over the mayo before broiling. You can also make these in a larger form.