Rinse fresh red chilies with water a couple of times. Then, wipe dry each red chili with a kitchen cotton napkin.
The chilies have to be completely dry before you proceed with stuffing them with the pickle masala.
Remove the crowns from the red chilies. Then, with a knife make a slit on each red chili. Ensure that you slit the chili only one side and keep it whole.
With the back handle of a small spoon, scrape off the seeds and pith from each red chili.
Once you remove the seeds and pith from each red chili, cover them and keep aside.
Making pickle masala
Take mustard seeds, fennel seeds and fenugreek seeds in a small grinder jar or coffee grinder.
Powder the spices coarsely. Do not make a fine powder.
Take the coarsely powdered spices in a mixing bowl or pan. Add dried mango powder, turmeric powder, asafoetida and salt.
Mix very well with a spoon.
Making red chilli pickle
Add 4 to 5 tablespoons mustard oil in the coarsely ground pickle masala. This is to moisten the masala so that it can easily be stuffed in the red chilies. Mix very well.
Take a portion of the pickle masala and stuff in each red chili.
Stuff the pickle masala in all the red chilies.
Now, place each stuffed red chili in a clean glass jar or ceramic jar. You can either sterilize the jar or keep it in sun with its lid for a couple of hours. Add any leftover pickle masala in the jar.
Now, pour 1 cup mustard oil in the jar.
Close the jar with its lid. Then shake the jar, so that the mustard oil coats all the red chilies.
Remove the lid. Place a muslin cloth on the mouth of the jar. Secure it tightly with a string or rubber band.
Place the pickle jar in the sun. During evening, remove the jar from your window sill, balcony or terrace and keep in a dry place in your kitchen. Next morning, keep in the sun again. Shake the jar everyday before keeping in sunlight. Keep in the sun for about 5 to 7 days.
To check if the pickle is done, taste the red chilies. If there is crunchiness in it, then some more days will be needed for the pickle to mature. The red chilies should soften due to the heat from the sunlight.
Once the Red Chili Pickle is matured, you can start serving it with your meals, store it in the refrigerator or keep the jar in a cool dry place.
Notes
Use the larger variety of fresh red chillies. Ensure that you don't end up using an overly spicier or hotter variety of chillies as then the pickle will become very spicy.
Do not replace mustard oil with any other oil. Also, you do not need to heat mustard oil.
Make sure that the spices are fresh, within their shelf-period and without any insects and moulds.
Note that the approximately nutrition info is for 1 pickled red chili.