Combine the vinegar, sugar, and salt in a small saucepan over medium heat. Bring to a boil and simmer for two minutes, until the sugars have dissolved.

In a small bowl, pour the pickling liquid over the onion rounds. Allow to sit for 10 to 15 minutes. Remove onions from liquid and pat dry. Transfer to a jar or Tupperware container, toss with a touch of olive oil and a sprinkle of salt and store until time of serving. Hold onto the liquid for another use.