Wash the mangoes and green chillies. Wipe them with a clean cloth/paper towel and let them stay outside for about an hour until no moisture is left.
Then cut the mangoes and chillies into medium-sized pieces.
Take two mixing bowls. In one, put the chilli pieces and in the other, put the mango pieces.
On medium heat, toast fenugreek seeds, mustard seeds and cumin seeds till the fenugreek seeds change colour and the mustard seeds start crackling.
Remove and let them cool. Once cooled, grind to a coarse/fine powder (your choice).
Divide this powder into two parts. Mix one part with the mango pieces and the other with the chilli pieces.
In another big mixing bowl, combine all the spices: Fennel Seeds, Fenugreek Seeds, Onion Seeds, Black Peppercorns, Turmeric Powder, Red Chilli Powder, Salt, and asafoetida.
Add mustard oil and give it a good mix.
Now divide this mixture also into two parts. Mix one part with the mango pieces and the other part with the chilli pieces.
Mix well.
Transfer them into two separate glass/ceramic jars with airtight lids.
Keep them in the sun for an entire day. At the end of the day, give them a good mix with a clean, dry spoon, close the lid tightly and keep them inside.
Repeat this step for another week.
After a week, the pickle is ready to be consumed.