Put the mustard, fenugreek and fennel seeds in a pestle and mortar. Crush to a coarse mix then set aside.
Heat the oil in a large frying pan or kadhai over a medium heat. Add the asafoetida and the crushed spices and fry for a few seconds, then add the ginger and fry for a minute.
Add the squash, chilli and ground turmeric, stir well and fry for 5 minutes. Add 3 tbsps water, season to taste and lower the heat.
Cover and continue to cook for 20 minutes, or until the squash is cooked, stirring halfway through. Crumble over the jaggery and the dried mango powder. Stir well and continue cooking for a few more minutes.