I love having pickled red onions on hand. They elevate any sandwich, eggs or cheese and crackers snack to something that feels elegant. I like to make a batch at the beginning of the week and throw them on anything I can think of.
In a medium bowl, whisk together vinegar, water, salt and sugar until fully dissolved.
Stir onion and jalapeño slices into pickling liquid. Toss to make sure they are coated with the liquid; they will not be fully submerged.
Cover and refrigerate for at least 30 minutes and up to 24 hours.