Thinly slice 1 red onion
Pack the onion slices into a clean jar
Add 1/2 cup vinegar
Add 1/2 cup water
Add 1 tablespoon sugar
Add 1 teaspoon salt
Stir until the sugar and salt dissolve
Pour the liquid over the onions
Press the onions down so they are fully submerged
Let cool to room temperature
Cover and refrigerate
Wait at least 1 hour before using
Store in the refrigerator for up to 2 weeks