In a bowl mix vinegar, water, salt and sugar (peppercorn if using). Mix until the sugar and salt is totally dissolved.

Add the thinly sliced onion into the mason jar or any glass jar.

Now add the vinegar mixture to the mason jar. Keep it a room temperature for 1-2 hours before refrigerating.

Pickled onion can be used for weeks when refrigerated. Top on tacos, sandwiches, and salads.