2 tablespoons sugar
1 tablespoon kosher salt
1 medium red onion, halved and thinly sliced
Bring the vinegar, 1/4 cup water, the sugar and the salt to a boil in a small saucepan. Remove from the heat and let cool for 10 minutes.
Put the onion in a medium bowl, pour the vinegar over, cover and refrigerate for at least 4 hours and up to 2 days before serving.