Preparation Whisk first 3 ingredients and 1 cup water in a small bowl until sugar and salt dissolve. Place onion in a jar; pour vinegar mixture over. Let sit at room temperature for 1 hour. DO AHEAD: Can be made 2 weeks ahead. Cover and chill. Drain onions before using.
Preparation Whisk first 3 ingredients and 1 cup water in a small bowl until sugar and salt dissolve. Place onion in a jar; pour vinegar mixture over. Let sit at room temperature for 1 hour. DO AHEAD: Can be made 2 weeks ahead. Cover and chill. Drain onions before using.