Sterilise the jars by thoroughly washing with soap and water then placing in a pre-heated oven at 120°C for 15 minutes.
Add the butter to a large fry pan on medium heat. Once the butter has melted add the thinly sliced brown onions.
Sauté the onions down until they become transparent, add the crushed garlic.
Add the red wine vinegar and balsamic vinegar and stir through followed by adding the brown sugar, whole grain mustard, mustard seeds and star anise.
Turn the heat down and allow to simmer for 12-15 minutes or until the liquids have reduced to a sticky sauce. Add a good pinch of sea salt.
Spoon the hot relish into the pre sterilised jars and place the lid on. Allow to cool down on the bench followed by placing in the fridge.