On third day, place sun dried mango pieces in a large bowl (I use a stainless steel mixing bowl). Avoid using a glass bowl for safety since we are going to pour hot oil later. Sprinkle the salt, red chilli powder, turmeric powder, fennel powder and mustard powder.
Next, add mustard oil to a sauce pan and warm it up on low heat.Take off the heat and to the warm oil, add hing powder, kashmiri chili powder and the whole spices (nigella seeds & fenugreek seeds). Give a stir and immediately pour it over the mangoes.
Using a spoon, mix very well such that all the mangoes are coated in oil and spices. The achaar will have a very strong smell and the masala will have a bitter taste right now but that's expected. At this point you will also feel that the oil is extra but as the pickle will cook in sun, the spice powders that we added are going to absorb the oil and swell. Also the mangoes need to be coated in oil at all times during fermentation to avoid bacterial formation.
Pour the mango achar in a tall glass jar leaving 2 inches space from the mouth of the jar. Tie a muslin cloth, malmal or a cheesecloth on the mouth of lid. Don't put the lid since we want the moisture of mangoes to escape during fermentation. If you will put the lid, the moisture trying to escape as the pickle heats up in sunlight will be trapped inside. Hence, your achar won't get to breathe and this will lead to mold formation.
Allow the achar to stand in full sun for next 8-12 days( this time will depend on the strength of sun in the area you live). Shake or stir the achar once or twice a day using a clean, dry spoon. Don't use hands.
At the end of sun fermentation, the skins of the mangoes would be brownish and the strong, bitter taste of spices will go away. The mangoes will be soft and pulpy. It won't be pungent, rather will have a tangy and spicy flavor. Your mango pickle is ready.