Quarter the lime halves and layer them in a large sterilized screw jar, sprinkling salt over each layer. Close the jar tightly and steep in a warm place for 10-14 days, until the limes are brown and soft.
2. Combine the chili, mustard, fenugreek and turmeric powders in a small baking bowl and add to the limes form. Mix well. Close the jar and let stand for 2 days.
3. Place the limes in a heatproof baking dish. Heat the oil in a heavy frying pan. Add the mustard seeds and asafetida and heat the oil, stirring throughout, until it begins to smoke. Pour the oil over the limes and mix well.
Cover and cool. Pour the cold lime pickle one more time into the screw-top jar and seal tightly. Stand in a sunny spot for 1 week before serving.
Tip If you are sure the screw-top jar is oven-safe, you can pour the hot oil directly into the jar in step 3 and mix with the limes.