Photograph: Martin Poole
Trim the coriander, leaving at least 5cm of the stalks to boost the flavour. Wash well, shake off any water and transfer to a food processor.
Rinse the mint and strip the leaves from the stalks – you should end up with about 30 leaves.
Put the mint leaves in a food processor with the coriander; add the chillies, ginger, spring onions, lemon juice, sugar, salt, yogurt and 3 tablespoons of cold water.
Blend to a bright green purée. Spoon into a serving bowl or cover and chill in the fridge.
Get ahead: it will keep for 3 days, but stir well before using.