Over a medium-low heat, add your oil, followed by your thinly sliced onions. Stir to coat and let them cook for around 25-30 minutes until soft
Increase your heat to medium-high, add your mustard seeds, vinegars and coconut sugar. Stir to combine and then cook for a further 25-30 minutes until the liquid has reduced. Add salt and pepper to taste. If there is excess liquid, at this point you can add your tapioca starch if you'd like to thicken it further.
Use a hand mandolin to thinly slice your onions in a fraction of the time it would take to use a knife!