This is something you must have on hand in the summer. On the other hand these really pump up a sandwich anytime of the year.
1 1/2 cups red wine vinegar
1/4 cup water
2 tablespoons sugar
1 tablespoon kosher salt
1 medium red onion, peeled, halved and thinly sliced
Bring vinegar, water, sugar and salt to a boil in a small saucepan over medium heat. Remove from the heat and let cool for 10 minutes. Put the onions in a medium bowl, pour the vinegar over, cover and refrigerate for at least 4 hours and up to 48 hours before serving.