Onions that are sliced paper-thin soften quickly when pickled. The vinegar cuts the astringency of the onions and brings out their natural sweetness. These are wonderful atop veggie burgers, tossed in salads or as a garnish for various crostini.
In a small bowl, add all the ingredients and toss together well. Gently massage the onions so they absorb the liquid. Allow them to pickle for at least one hour or, for best results, let the onions pickle overnight.
Store in a sealed container in the refrigerator, where they will keep for up to one week.