TOSS: Cut the onion through the root, discarding the top and skin. Slice each onion half into thin half moons, about 1/16″ – 1/8″ thick. No thicker! Add the sliced onions to a non reactive bowl, along with the lime juice and a sprinkle of fine sea salt. Toss until the onions are evenly covered in the lime and salt.
PICKLE: Let the onions sit at room temperature until they begin to soften a little, about 30 minutes or so, stirring them every 5 – 10 minutes or so. Add the dried oregano, if using. Toss the onions again to coat, then transfer them to a glass jar (or two). Pour over the liquid from the bowl, too. Gently press the onions down with the back of a spoon or fork so that they’re immersed in the lime juice. Refrigerate for at least 15 minutes or so before using – they’re even better once they’ve hung out in the fridge for a few hours.
STORE: Keep in an airtight container in the refrigerator for up to a week.