Whisk all ingredients together in a small heavy-bottomed saucepan and place over medium heat. Bring to the boil, reduce heat to low and continue cooking at a simmer, stirring another 10 minutes. Remove from heat and allow to cool completely.
Pour into a glass jar and store uncovered in the refrigerator for 8 hours or overnight. This allows the flavours to blend and reduce any sharpness from the vinegar.