Add the oil to a large saucepan. Heat over a medium-low heat. Add the chopped onions and stir to coat in the oil. Fry gently for about 7-8 minutes until buttery soft.

Season with salt & pepper, add the mixed spice and mustard seeds. Stir. Then add the sugar and balsamic vinegar. Bring to a gentle simmer. Cook low and slow for 10 mins, stirring occasionally until beautifully caramelised and deep purple/brown in colour. Add the water to loosen the mixture and simmer and reduce for another 10 minutes until left with a slightly wet mixture. Remove from the heat and leave to cool.