2 medium red onions, peeled, halved and thinly sliced
1 ½ cups distilled white vinegar
1/2 cup water
2 tablespoons sugar
1 tablespoon kosher salt
Put sliced onions into a bowl. Combine vinegar, water, sugar and salt in a small sauce pan. Bring to a boil. Lower heat to a simmer and cook just until salt and sugar dissolve. Pour hot vinegar mixture over onions and stir.
Let the onions cool for 30 minutes or so. Cover and store in the fridge for at least 45 minutes and up to 2 weeks.