Take mustard seeds, lightly roast and powder in mixer or with a rolling pin, measure out tsps.
Heat the oil. Just before it starts smoking, remove from heat. Add the salt, chilli powder, turmeric powder, mustard powder, fenugreek powder, asafoetida. Stir well with a dry spoon.
Place the diced raw mango in a nicely dried large bowl. Pour the oil with the spices on top of this and stir well to mix with a dry spoon.
If you have an earthen pot, keep it in the sun or microwave it until it is thoroughly dry and sterilised. If you dont have an earthen pot, use a glass bowl or ceramic bowl.
Add the raw mango dice with spices into this. Cover with a tight fitting lid and then tie a muslin cloth around this (optional).
Keep this in a clean, dry place in the kitchen. Once everyday, give it a good stir and mix up for the next 3-4 days.
No need to keep in sun. After 4-5 days, fill into 4-5 clean and dry glass bottles or ceramic jars. Place the pickle in refrigerator, where it should last for 2 months or so. Always use a clean, dry spoon while serving.