Put the mango pieces and the chickpeas in a steel or glass bowl and add the salt, chilli powder and turmeric powder. Stir gently, to coat the pieces, and keep aside.
Heat an iron kadai/ wok/ skillet and dry roast the mustard and fenugreek seeds (do not brown), over medium heat, till they start giving off an aroma. Take them off the heat, allow to cool, and then grind the seeds to a fine powder in a mixer/ blender.
Heat the sesame oil in the same kadai/ wok/ skillet but do not let it reach smoking point. Add the asafoetida to this and take the oil off the heat. Add the powdered mustard and fenugreek to the oil mix and allow it to cool.
Pour this oil over the mango pieces and mix well with a spoon, using a folding motion of the hand. Transfer the pickle to a sterile glass jar, and store at room temperature stirring the contents once a day, for about a week.
The pickle can be eaten in about 3 weeks and should keep without refrigeration. This recipe makes enough pickle to fill a medium glass jar. For larger quantities, increase proportions of the ingredients as required. Serve with rice and yogurt for the best combination.