Put the mustard seeds, fenugreek and fennel seeds in a mortar, crush coarsely, then set aside. Put the oil in a large frying pan or kadhai on a medium heat. Add the crushed spices and the asafoetida, and fry, stirring, for a few seconds. Add the ginger, fry for a minute, then stir in the squash, chilli and turmeric, and fry, stirring, for five minutes more.
Add 50ml water to the pot, season with salt to taste and bring to a boil. Cover, turn down the heat, and leave to cook for 20 minutes, stirring once halfway through. Add the jaggery, crushing some of the pieces a little, then add the mango powder, stir well and serve warm with chapatis or naan.