Peel and finely slice the red onion. Separate out the rings then add the onions to a clean jam jar or mason jar.
Add all the remaining ingredients to a small pan and bring to a boil then turn off the heat.
Carefully pour the hot liquid over the onions, pressing down the onions with a spoon until they are completely submerged.
Fasten the lid and leave aside to cool. These will be ready to serve once cool, in about 30 minutes.
Leftover pickled onions can be kept in the fridge for a week.
