Put the red onions, lime zest, oregano, salt, sugar, and black pepper into your mason jar. Pour over the lime juice and vinegar.
½ teaspoon lime zest, 1 sprig fresh oregano, 1 tablespoon granulated sugar, 1 teaspoon kosher salt, ⅓ cup lime juice, ½ cup apple cider vinegar, ½ teaspoon black pepper