Wash and wipe the mangoes with a cloth and cut them into pieces. Clean the pieces with a soft cloth and spread them over a dry cloth while you work on the rest of the pickle preparation.
Wash the pickle jars and sun dry the jar till completely dry with absolutely no moisture.
Sun dry the mustard seeds and grind the sun dried mustard seeds to a fine powder. Set aside. Peel the garlic and set aside.
Take a large wide vessel or bowl and add red chili powder, mustard powder and salt and mix well with clean, dry hand. Add methi seeds, garlic cloves and 3/4 cup oil and mix well.
Add the mango pieces and mix well with your hands such that the mixture coats the mango pieces. Place the spice coated mango pieces along with some of the spice mixture and add to the jar.
Any remaining spice powder should be added to the jar. Pour Oil on top and cover the pickle jar with the lid. Tie a cloth over the lid and keep it in a moisture free area up to 3 days.
On the 4th day, open the pickle jar and use a clean dry ladle and mix the contents carefully. Taste the mixture and check if its salty. If required add more salt.
Pour more oil on top as the pickle needs to be covered in thin layer of oil. Cover the jar with lid and tie a cloth over it. Let the pickle marinate for another 4 days.
After 4 days, mix the pickle carefully with a clean and dry ladle. The mango pickle is ready for consumption.
Serve this pickle with hot steamed rice.