Slice the red onion as thin as possible using a mandolin, then place the slices in a shallow bowl or wide-mouth mason jar. Set the onions aside.
In a small saucepan -- bring water, apple cider vinegar, salt, peppercorns, red pepper flakes, and sugar to a boil.
Stir until the sugar and salt dissolve. (Try not to stand over this/breathe it in as it boils - the vinegar will be pungent!)
Pour the hot liquid over the sliced onions carefully then press the onions down to make sure that they are completely submerged in the brine.
You can slide the handle of a spoon or fork into the jar to pop any air bubbles.
If the onions are not going to be eaten right away, let them sit at room temperature until they cool off. This should take 30 minutes - 1 hour. Once cooled, cover the onions and put them in the fridge.
🙋♀️ Frequently Asked Questions How long do pickled red onions last? Pickled red onions will be good in the fridge for about 3 weeks, but once you taste them I doubt they will last that long.What do you eat pickled red onions with? You can eat pickled red onions on so many things - tacos, burgers, carnitas, off of a fork, on a bagel with lox and cream cheese - so many things!Which vinegar is best for pickling? I love using apple cider vinegar for this recipe, but you can also use distilled white vinegar if that is all you have. Just check the label and make sure that it says 5% acidity. If it says 4%, adjust the quantities and use a little more vinegar and a little less water.Are pickled red onions good for you? Yes! Red onions are one of the most nutritious of the onion "family" for you because they loaded with vitamin B9 and folate which can improve heart health - so they are tasty and good for you 🙂Can you reuse pickled red onion brine? You could get away with using the brine a second time, but the salt/sugar/vinegar/flavor will be a little watered down as the onions will release some liquid. Just bring the brine to a boil again and pour it over fresh onions, though use slightly less onion because you will have less brine. I often do this, but the first batch is usually the best.🍳 Serving Suggestions & Uses Served pickled onions cold if you can - at least that is how I like them - some people like them room temperature. (Cold is my favorite way to enjoy them.)I love them on top of a taco, burger, a bagel with lox and cream cheese, carnitas - so many things! What is your favorite way to enjoy pickled onions?🥫Storage/Shelf Life/Reheating Pickled onions need to be kept in the refrigerator - this is not a canning recipe and these are not shelf-stable. They will last up to three weeks.📋 Tips - If you don't have apple cider vinegar, you can also use distilled white vinegar if that is all you have, just be sure check the label and make sure that it says 5% acidity. If it says 4%, adjust the quantities and use a little more vinegar and a little less water. - If you want to make pickled onions that can be canned and preserved, this website has a lot of safe canning recipes. I personally love the Balsamic Onion Jam and the Broiled Pickled Onions on the site.