1/4 cup white wine vinegar
2 tablespoons sugar
1/2 teaspoon kosher salt
1 small red onion, halved and thinly sliced
Combine first 3 ingredients in a small saucepan over medium-high heat. Bring to a boil, stirring until salt and sugar dissolve. Remove from heat.
Add onion to vinegar mixture. Let sit at room temperature at least 2 hours, stirring occasionally.