In a large pan, cook the peeled and diced ash gourd along with turmeric powder, chilli powder, salt and water until tender.
While the ash gourd is cooking, grind grated coconut, green chillies, cumin, turmeric and water into a fine paste.
Whisk the yogurt lightly and keep it aside. Once the ash gourd turns tender, turn the heat to the lowest setting and pour in the ground coconut mixture. Mix well and let it cook for a minute before pouring in the yogurt. Stir continuously for 2 minutes and remove from heat.
Temper mustard seeds, fenugreek seeds, turmeric powder, dry red chillies and curry leaves in coconut oil and pour it over the curry. Mix gently and serve this delicious curry with steamed rice, pickle and pappadum.