Put the limes into a sterilized jar and cover with the salt. Dry-fry the fenugreek and mustard seeds in a small frying pan, and then grind them to a powder. Add the ground seeds, chilli powder and turmeric to the limes and mix well.
Heat the oil in a small frying pan until smoking, add the asafoetida and fry for 30 seconds. Pour the oil over the limes and mix well.
Cover the jar with a clean cloth an d leave to mature for 10 days in a bright, warm place.
Transfer the pickle to a tightly covered container and store for up to 2 months.