Heat the vinegar mixture in a small saucepan, stir together the vinegar, water, salt and your desired amount of sweetener. Cook over medium high heat until the mixture reaches a simmer. (Or alternately, you can heat the mixture in the microwave.)

Combine in a jar. Place the thinly sliced onions in a mason jar. Pour the hot vinegar mixture over the onions, screw on the lid, and shake the onions briefly until they are evenly coated with the vinegar mixture.

Marinate. Make sure the lid is on tightly and let the onions marinade for 30 minutes upside down on a towel.