Chop the spring onion, garlic and chilli.
Heat the oil in a heavy-based saucepan over medium heat.
Add the onion, garlic and chilli and gently cook for 5 minutes until the onion has softened.
Add the mustard seed, stir for half a minute then add the sugar, raisins and vinegar. Let it bubble for a few seconds.
Cover with a lid, turn down the heat and gently simmer until the liquid has evaporated and the onions are very soft and sticky.
Season with salt and pepper then transfer to a sterilised jar or a small bowl.