Quick and easy to make using only a few ingredients, pickled red onions are tart, sweet, and have a pleasing amount of heat that can be modified depending on the ingredients. Pickled onions are very versatile, and although we mostly make them for pulled pork (carnitas) tacos, they also go very well with salads, sandwiches, and burgers.
There are dozens of pickled onion recipes on the Internet, but almost all the recipes will include some type of acid (vinegar, citrus, etc.) to marinate the onions. There are no hard and fast rules when choosing ingredients - once you've made the basic recipe, try upping the heat with jalapenos (or even habeneros!) when you make them the second time. If you used vinegar the first time, try citrus the next time.
Once you've chosen the marinating acid, the two processing methods are either on the stovetop where the brine is heated then poured over the ingredients; or the soaking method where the onions are soaked in very hot water, then drained before the rest of the ingredients are added. The soaking method helps to remove some of the harsh taste from the raw onions. Both processes work well, but we prefer the soaking method to the stovetop. The image below shows the pickled onions once they were blanched and about halfway through the marinating process.
Once the onions and other ingredients are added to the marinating brine, they're covered with plastic wrap and put in the refrigerator for 24 hours. The next day, just drain the marinade and you're good to go.
Here are two videos that show how to make pickled onions. The first video uses the soaking method and the second video is from the pros at America's Test Kitchen, and they use the stovetop method.