Slice the blossom end off of a red onion, lay the now stable, cut surface on your cutting board. Slice the onion in half and remove the peel. Holding the root end, slice the onion into thin half moons. Put them into a heat-proof bowl. Bring the vinegar and salt to a boil and quickly pour over the onions. Wrap the bowl tightly with plastic wrap and let sit at room temperature for at least an hour, removing the wrap and tossing with tongs every 20 minutes, then re-covering, until the onions are bendable. When they reach that point, load them into a jar, pour the pickling liquid over them and screw a lid tightly in place. Refrigerate until well chilled.

Store tightly covered for up to a month in the refrigerator.