Wash the green chillies and dry them thoroughly. Make one slit on each chilli and set aside. Heat 1/4 of the oil and add the mustard seeds and fry them till they crackle. Take them off the stovetop and mix with the salt. In the same pan, brown the asofetida till it starts turning brown. Remove and add to the mustard/salt mixture. Stuff this mixture in the green chillies and save any remanining mixture. Heat the remaining oil and add the green chillies and any remaining stuffing. Fry on a very low flame till all the chillies turn white in color. Remove and cool. Store in an airtight container. This will keep good for months.