I got the recipe for the onions right from one of the ladies at Pollo Lucas.
Slice up a red onion very thinly. Add white vinegar, add salt. The amount of vinegar and salt depend on how much onion you're making. There's no exact ratio, but there should be enough liquid to almost cover the onions. I add the salt a little at a time, and I keep tasting it - when you hit the right amount you'll know it. I make them in a tupperware bowl, cover them give 'em a good shake and then put them in the fridge for a while. They'll need at least a couple of hours to be ready, but they last in the fridge like that for a couple of weeks at a time.
The last time I made them I added about a dash of sugar also - just to take out a little acidity. That's all there is to it!