1/4 cup red wine vinegar
1 tablespoon kosher salt
1 tablespoon sugar
1/2 red onion
In a medium saucepan, bring vinegar, 1/2 cup water, salt, and sugar to a boil. Meanwhile, slice onion into thin half-moons and pack into a pint jar. Pour in hot liquid, then let cool. Chill overnight.
Make ahead: Up to 4 days, chilled.