The large onions were usually lacto-fermented and extremely easy to do at home. I do it regularly because they are delicious and impossible to find anywhere. Directions on how to do it are online, but if you are really interested in improving your over all health, I would recommend getting this book and start fermenting lots of different items.

Google Lacto-fermented onions for recipes.

All you need to ferment is a vessel to hold the onions and a saltwater brine. Nature and time takes care of the rest. I would recommend buying a fermenting vessel with a valve. While not needed it definitely makes it easier to control the fermenting environment inside the vessel. Something like this:

Of course you can just pickle with vinegar too, but it will not taste as good, nor deliver the same health benefits of the fermented variety.