In a large dry bowl, add the cut mango. Air it out for 5-10 mins to let any residual moisture out.
Heat a thick iron kadai, add the mustard seeds. let it splutter well , transfer to a plate, add fenugreek seeds (methi seeds) and let that aroma waft out (3-4 seconds), transfer to the mustard plate. Let it cool, powder fine.
In the same dry pan, fry the salt (too!!) , red chilli powder and turmeric separately. This is to ensure there is no moisture from the dry spices too (I follow this method for ALL my pickles to increase the shelf life). Add these 3 to the chopped mango pieces.
In the hot kadai, heat the sesame oil and let it smoke for 10 seconds. Switch off heat and let it remain in the hot pan.
Now mix the raw mango, spices, salt and the spice powder well. Add the spice powder in batches, adding only enough to coat. Else the fenugreek-mustard pungency can overtake the tart mango taste.
Now cool the oil well and add to the pickle.
Cooling the Oil before adding to the pickle is EXTREMELY Important, else the pickle will spoil in few hours.
Add lemon juice in the end.
Mix well. Let it sit for 1-2 hours before serving.
Use a dry spoon whenever you access the pickle and store in an airtight glass jar and refrigerate it for further use.