Add the thinly sliced red onion and jalapeno slices to a jar (I used a 20 oz jar), along with the pepper corns and sugar. Pour over the hot water and apple cider vinegar until they are covered by the liquid. Stir, seal the lid and place in a sunlit spot on your counter.
The pickled onions will be ready an hour later when they are vibrant in color and have a bold pickled taste. Refrigerate in a sealed container for up to 1 week.