Heat oil in a kadai on high flame till smoke comes out. Now add hing and heat for 1 min.more. Cool down naturally.
Clean and wash all chillies properly then wipe it off with kitchen towel ; keep aside.
Peel off garlic, slice thick garlic cloves in 2-3 according to size. Keep aside.
Wash, peel off and finely chopped ginger ; keep aside.
Cut chillies in bite size or 1/2" size ; keep aside.
Take a big size open steel, wooden or food grade plastic pot. Mix all dry ingredients mustard, coriander, kashmiri chilli, fenugreek, fennel, turmeric powder together. Now add carom, fennel, fenugreek , onion seeds , black salt, dry mango powder, salt and mix very well.
Now add chillies pieces, ginger, garlic and stir up several times so spices mixture coat evenly on chilli, garlic and ginger.
Keep pot in full sunlight ; stir well 4-5 times during daytime.
After 1 to 2 days according to sunlight exposure add oil in pickle so spices will coated properly and pickle will not look dehydrated too much.
Keep pickle in sunlight for 4-5 days according to sunlight quality and stir up properly 3-4 times every day. This process could take less days if you use large size open tub. I have done same.
According to tub size and hottness of sunlight put pickle in a dry airtight jar (If you sunlight is good it will take 3-4 days for this process) Tie muslin cloth on the neck of jar.
Keep it like this in sunlight for another 2 days. Its moisture will come out in two days. Now take out muslin cloth and put the lid.
Before start rainy season keep jar in sunlight to keep pickle safe for longer period. Always use wooden spoon for stirring. Use clean and dry spoon to take out pickle from jar.
Serve this mouth watering pickle with any type of poori, paratha and rice.