Rinse the onions well. Trim the root end of each onion, leaving the core intact. Squeeze the onion gently from top end to remove tough outer skin. Discard the skins. Let the onions dry completely, overnight.
Using a mortar and pestle crush the spices/seeds (mustard seeds, fenugreek seeds, asafoetida, and whole coriander seeds) gently.
Add the crushed spices, Kashmiri chili powder, salt, onions and 1/4 cup oil in a mixing bowl. Mix well.
Transfer the mixture to a glass jar with lid. Add the remaining 1/4 cup oil.
Cover the glass jar with a lid and keep it in direct sunlight (in summer) for 4 days. If it isn't very bright and sunny keep the jar for at least 7 days.
To check for doneness, open the lid and check, it shouldn't smell like raw onions. The oil should be visible from the top.
Store in the refrigerator to retain crunchiness.