Combine all ingredients except for the the onion into a small saucepan. Gently heat to a gentle simmer
Thinly slice the red onion (I like to use the slicer on the food processor to get super thin). Add the sliced onion to a 400 ml jar.
Pour the contents of the saucepan into the jar. Press the onion down with a spoon until it is entirely covered by the pickling liquid. Cover and let cool for 30 minutes. It will then be ready for serving.