Thinly slice the red onions into rings.

Combine the rest of the ingredients to make the pickling juice. Place the onions in a sterilised jar, and fill the jar to the top with the vinegar mixture.

Seal and refrigerate for at least 1 day.

Cheat: If you are in a hurry to get the onions made, pour boiling water over the raw onions, wait until cooled, drain the water, then top with the vinegar mixture and rest for a few hours.

Will safely keep in the refrigerator for 2 weeks or more.