Add the plums to 200ml boiling water and boil for 15 minutes until soft inside.
In a sink, place a colander over a large bowl and transfer the plums with their liquid. Let the liquid run through into the bowl.
Use a whisk to rub the plums against the bottom of the colander. The skin will stick to it and the flesh will fall through.
When there is no more flesh coming off, remove the colander and whisk the contents of the bowl to incorporate the pulp.
Once smooth, return the puree to the pot. Add 1 tbsp sugar and simmer over medium heat for 15 minutes until slightly reduced.
In the meantime, blitz together herbs, garlic, green chili with 1 tbsp of oil and 1 tbsp vinegar into a smooth paste.
Once the sauce has thickened, add the paste and stir to combine.
Add a drizzle of lemon and 1/2 tsp salt and taste to adjust for sweetness.
Transfer to an airtight container and let cool at room temperature.
Keep in the fridge for up to two months.