Place the sliced onion in a tall mason jar.
In a medium saucepan bring the remaining 5 ingredients to a boil, reduce to a boil and stir until the sugar and salt dissolve, about 1 minute. Allow the mixture to cool.
Pour the liquid over the onions, cover, and refrigerate. Onions are ready when they’re pink and tender, about 1 hour. Refrigerate onions will stay fresh up to a month.