Thinly slice 1 red onion
Pack the onion slices into a clean jar
In a small saucepan, combine 1 cup vinegar, 1 cup water, 1 tablespoon sugar, and 1 teaspoon salt
Heat the mixture until the sugar and salt dissolve
Pour the hot liquid over the onions until fully covered
Press the onions down if needed to submerge them
Let the jar cool to room temperature
Seal and refrigerate for at least 1 hour before using
Store in the refrigerator and use within 2 to 3 weeks