This is the classic accompaniment to roast lamb, and insanely easy to make.
Wash the mint and chop it as finely as you can. Put into a bowl and stir in the vinegar and sugar. Refrigerate for at least half an hour for the flavours to develop.
You can keep it in a jar in the fridge for a couple of weeks.
If you want a thicker sauce, you can add a teaspoon of powdered arrowroot dissolved in a tablespoon of water.