My wife made this recipe up, after eating something similar, but if you are making brisket tacos, it is the most wonderful accessory:

Ingredients

Chop the red onions into rings and put them into a container of vinegar. Add the sugar and store the mixture in your refrigerator to marinate and pickle, the longer the better.

Pick out the rings and place them on your brisket taco. Eat.

Variations

Pickled onions are a favorite in the Von Doom house, too. @littlewhirl played around with a similar starting point and we use this brine instead:

The apple cider vinegar is a tangier but also more subtle, keeping more of the onion flavor there. And using a partial salt water brine lets it work nice for quick pickles, but also for slightly longer engagements. Leading to the most important aspect:

PICKLE ALL THE THINGS

Seriously. All the things. Pickle your fridge. Pickle your waffles. Make. Lots. Of. Pickles.