Wash the mangoes well and dry them thoroughly. I left the washed mangoes covered with a dish towel on the counter for couple of hours to completely dry them.
Prep all the spices: Dry roast (without oil) sesame seeds until lightly browned on medium low heat. Set aside.
Dry roast fenugreek seeds and mustard seeds separately.
When all the spices are completely cool, grind each one separately; sesame seeds can be made into a coarse powder – too much grinding can make sesame paste, so keep checking. Mustard seeds tend to get bitter if ground too much, so be mindful of that.
Heat oil in a small saucepan until very hot, turn off the heat and let it cool completely.
While the oil is cooling, start chopping the mangoes. Cut the mango into half lengthwise, this gets a little tricky if the seed is tough, so try to find the exact spot where the seed ends. Scoop out the seed and discard. Cube the mango into small bite size pieces. Try to make sure that each piece has the peel. Pat dry the pieces with a clean kitchen towel to make sure that there is no moisture on the pieces.
Take all the pieces into a large non-reactive container. Add sugar, salt, pickle chili powder, all of the roasted fenugreek and mustard powders, 1 cup of sesame powder (reserve the rest for later) and the cooled oil. Mix everything together.
At this point the pickle is a little thick, but as it sits the mangoes will ooze out their juices and will thin out over time. Cover tightly and let the pickle rest for 24 hours.
Taste the pickle and add more salt/ sugar/ chili powder or the reserved sesame powder if needed and let it sit for another 24 hours. If it tastes perfect after the 1st 24 hours then simply put the pickle into clean glass/ porcelain jars.