Wash and finely chop the lemons, discarding the pips, then deseed and finely slice the chilli.
Heat the oil in a small pan over a low heat and add the mustard and cumin seeds. When they start to pop, throw in the curry leaves. Fry for a minute or so, until they’re nice and crisp.
Add the lemons, sugar, chilli and a pinch of sea salt. Turn up to a medium heat and cook for around 10 minutes, or until thick, sticky and the lemons have softened.
Leave to cool, then store in a sterilised jar in the fridge, where it will keep for up to a week. Delicious served with poppadoms, plain yoghurt and a red onion and tomato salad.