Wash and wipe the mangoes. Cut it into bite sized pieces working around the seed. It weighed approx., 1 kg without the seeds, roughly 3+3/4 cup when measured.
Heat a pan and dry roast fenugreek seeds, shaking the pan so that it gets roasted well. Take care not to burn the seeds. Transfer it to a bowl and to the same pan add the mustard seeds.
Dry roast for a few seconds while stirring with a ladle. Transfer both to a mixie jar and grind it to a coarse powder. Now add red chili powder, rock salt and grind it to a fine powder.
Transfer the cut mango pieces to a food grade container. Add the ground spice powder and stir well with a wooden spatula so that it coats all the mango pieces evenly.
Add the gingelly oil to the mango pieces and stir well. It will look dry and as though the oil is not enough. But cover the container and let the pickle sit for 2 days.
If possible keep shaking the container or stir using a clean spatula 2 or 3 times a day. After that, you can see the oil floating on top.
After a week of resting and letting the mango pieces soak in the oil and spices, they would have softened considerably well.
Avakkai pickle is ready to be served.
Transfer to sterilized, dry bottles or ceramic containers (barni) and pour the remaining oil so that they float on top.
Store in room temperature. It keeps well for a year. Use a dry spoon while using to serve.