Put a small pot or kettle of water on the stove to boil.
Slice onion super thin (you can use a mandolin if you have, otherwise use a sharp chef's knife) and put them in a sieve or colander in the sink.
In the container you'll be using the store the onions, combine the vinegar, sugar and salt. Stir to dissolve.
Pour the boiling water over the onions and let them drain.
Add the onions to your storage container and mix (or cover tightly and shake) to evenly coat.
Cover and refrigerate. They'll be ready in 30 minutes, but are better after a few hours and best after a few days.
Keep covered and stored in the fridge. Shake or stir every so often to distribute the liquid.