Wash and pat dry the chilies. Cut and discard the top stem and the entire green portion, then cut them into half. I discarded the seeds & veins of half the chillies,that's where the heat in the pickle comes from. You can remove seeds for all of them if you want. Layer the chilies on a wide, non reactive shallow dish, sprinkle a ¼ teaspoon salt.
Using your coffee grinder, coarsely pulse the mustard, fennel, nigella & fenugreek seeds separately (this is important).You do not need to make a fine powder, if few specks of whole spices remain, its okay.
In a small bowl mix the powdered spices together with turmeric powder, remaining salt and amchur.Taste the spice mix for salt, it should be slightly more salty than you think. Add 2-3 tablespoon mustard oil just so that the spice powder is moistened. This will be make it easy to stuff the chilies.
Wear gloves and stuff whole or each half of the chilies with 1-1.5 tablespoon of the achar masala.
Place the stuffed peppers in a wide glass dish (I use my pyrex). Add 200 ml of mustard oil, any spice mixture remaining and gently mix so that the chilies are coated in oil.
At this point, the achar will have a very strong smell and a bitter taste but that's okay. Allow it to stand in full sun for two days. Try to stir once or twice a day using a clean, dry spoon.
On the third day transfer the pickle into a glass or porcelain jar (do not use a metal container) ,taste and adjust the salt and top with remaining oil. Shake a little to mix.
Cover the mouth of the jar with a muslin cloth, tie with a string and let mature for seven to ten days in sun. (this time will depend on the strength of sun in the area you live).Shake the jar once or twice a day.
At the end of sun fermentation, the skin of chilies would have shrivelled and the strong, bitter taste will go away. It will be tangy and spicy! Store at room temperature for up to 2-3 months. Always use a clean spoon to serve the pickle.