Place all the ingredients in a small saucepan and bring to the boil over medium heat. As soon as the sauce starts to boil, reduce the heat and simmer for 5 minutes, or until the sauce has slightly thickened. Take the pan off the heat and leave it to cool.
Remove and discard the ginger and spring onion. Pour the sauce into a sterilised jar and keep in the fridge for up to 1 month.
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