Wash the mangoes and clean them with a dry cloth ensuring there is no moisture. Now, cut the mangoes into two pieces, remove the seeds and further cut into small cubes.
In a large bowl, add 2 tbsp salt and the mango cubes. Marinate overnight. The mangoes will release water.
Next day, drain the water from the mangoes. Add turmeric powder and mix well.
Now, spread the mangoes on banana leaves and sun dry for 3-4 days.
Heat a pan and roast fennel seeds. Allow it to cool. Once cooled, grind the fennel seeds. Keep aside.
In the same pan, roast mustard seeds. Grind the seeds to powder. Keep separately.
Heat 250 gm of mustard oil. Add red chillies and panch phoron. Saute for a while. Turn off flame and let the oil cool down.
Now, take a glass bottle, and fill it with the dried mango cubes.
Pour the oil into the bottle.
Add the mustard seeds and fennel powder.
Close the bottle and rotate it so that the ingredients mix well. Add remaining 250 gm mustard oil.
Keep the bottle in sunshine for 1-2 weeks. Your pickle is ready.