Pile the thinly sliced onions into a non metallic bowl or large glass jar.
Pour over the vinegar and add the salt & sugar. If you wish to include any of the optional extras below, add them at this point also.
Mix well, cover and refrigerate. They are ready to eat after 20 minutes, although in my opinion taste their best after 8+ hours in the brine.
Add 1/2 to 1 chilli in the initial stages.
You can either slice the chilli to incorporate it throughout the pickle, or keep the chilli whole, and just cut one slice through its length to allow the juice to seep out with out having the extra heat from the flesh and seeds when you come to serve.
Simply omit the sugar from the above recipe.
I find that if you make them sugar free, you want to let them marinade for at least 8 hours as otherwise they can be a bit too punchy!