Cook together sliced mangoes, turmeric powder, chilli powder, green chilli and salt with with 2 cups water. Bring it to boil and let it simmer, till the mangoes are cooked and reaches a mushy consistency.
Grind together coconut and cumin with 2-3 tbsp water to a smooth paste.
Add ground coconut paste to cooked mangoes and mix well. Let it cook for 5-7 mins on low - medium flame.
Remove the pan from the fire (to avoid splitting / curdling) and gradually add yogurt / buttermilk. Gently mix till no white traces are seen.
Bring it back to fire and cook on low flame for 5-6 mins, till steam comes from the back of the spoon. Do a taste test and add more salt, sugar (if required).
Heat oil in a small pan and crackle mustard seeds. Add curry leaves & dried red chilli. Fry for 1-2 mins and add to the curry.
Let the curry rest for 10-15 minutes before serving.
Notes
I used Alphonso mango for this dish. Back home we use Nadan Mango for this recipe, you can use any sweet variety mango. If using Nadan Mango, use 3-4.If using Yogurt, make sure you blend it in the mixer to make it smooth and remove any lumps. Those in Gulf can use Laban. I used full fat laban for this. Make sure yogurt / Buttermilk / Laban is at room temp, to avoid splitting of the curry.The curry has a well balanced sweet and sour flavour, so if the mangoes you use isn't sweet enough, add a tsp of sugar to balance the sourness.To reheat, keep the refrigerated curry at room temp for half an hr and MW for 30-40 seconds.