Marinate the mango slices with shallots, salt, sugar, turmeric powder and red chilli powder for 30 min.
Meanwhile, make the Bengali pickle masala. Lightly dry-roast Paanch Phoron, cumin seeds, dry red chilli and fennel seeds. When the spices give out aroma, then switch off the gas and transfer this into a small bowl. Allow the spices to come at room temperature. Grind to a slightly coarse powder. Keep aside.
Take a big bowl and add the marinated mangoes (along with the tangy marinade). Chop the Aampapad into small cubes and mix into the marinated mangoes.
Add 1 tsp of the Bengali pickle masala. (You may add ½ tsp more if you like the salad spicier.)
Add the mustard oil, Kasundi and the salad leaves into the bowl and toss well before serving.