Place the red onions in a 1 liter (4 cup capacity) sterilised glass jar.
In a small saucepan, heat the remaining ingredients until the sugar has dissolved then pour over the red onions, ensuring the onions are completely covered.
Place the lid on the jar and allow to cool before placing in the fridge for at least 24 hours before serving.
These pickles will last for up to 1 month in the refrigerator.