Crush the mint leaves and sugar in a mortar and pestle (or chop very finely, or process just until chopped)
Add the boiling water and leave to draw for 30 minutes
Add the vinegar and mix to combine
Set aside until needed
Notes
If you prefer the taste, replace the caster sugar with brown sugar, and white wine vinegar or balsamic vinegar in place of malt vinegar. Add a bite to the sauce with half a chopped chilli.