Wash and finely chop the lemons, discarding any pips, then deseed and finely slice the chilli.
Heat the oil in a small pan over a low heat and add the mustard, fennel and cumin seeds. When they start to pop, throw in the curry leaves (if using) and fry for a minute or so until the smell is wonderful.
Add the lemons, sugar, chilli and a pinch of salt. Turn up the heat to medium and cook for around 15-20 minutes, 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 month. Serve with poppadoms, plain yoghurt and a red onion and tomato salad as a snack, or on the side of any curry or rice dish.