There a few Latin American chicken places around Baltimore and DC that serve a chicken sammie that has what looks to be pickled red onions on top.
I have tried to make my own, assuming you boil water with vinegar and sugar and then steep the onions in it for a while.
It never comes even close to tasting as good as the palces I visit. Not only do they not taste as good, I also never get the soft texture and brillant red color these places do.
Any tips? Maybe a basic ratio rule?
Not sure if this is what you're looking for but something that I found to be extremely good is an onion sliced and then just put in with a jar of banana peppers..after a week or so it takes on the flavour of the brine and just tastes amazing!
Good luck finding what you're looking for
...or try this from a Rick Bayless book...
1 red onion peeled and sliced, and blanched in boiling salted water for 45 seconds.
then place in a small bowl and add:
1/4 teaspoon ground black pepper
1/4 teaspoon ground cumin
1/2 teaspoon dried oregano
2 halved garlic cloves
1/4 teaspoon salt
1/3 cup cider vinegar
water to barely cover
Let stand until onions turn bright pink.
I hope this works out well for you.