This recipe by Bon Appétit is so easy, you’re going to laugh.
If you don’t have red onions on hand, no worries! Substitute with white or yellow onion, and add pickled beet juice to the brine. I made 2 batches, one with red onion, and the second with yellow onion with added beet juice. Both were crunchy, tangy and delicious, but the beet juice batch was much prettier.