Peel the mangoes and cook the peeled mangoes with salt and 1 -1 1/2 cups of water for 10 minutes.
Grind together coconut, jeeraa, garlic, small onion, green chilli and turmeric powder with 1/2 cup of water to a fine paste. Add this and 1/2- 1 cup of water to the mangoes. Check salt and add more if required.
Cook for 5-7 mins on low flame. Make sure that it doesnt boil. Remove from fire.
Heat oil in a pan and splutter mustard seeds. Add sliced small onions, red chilly & curry leaves. Fry it till the onion becomes golden brown. Pour this over the curry. Let it rest for a while for the flavors to settle in. Serve with rice.
Notes
I used coconut oil. The gravy of this curry has a medium thick consistency, so adjust the amount of water accordingly. I think back home this is made with a type of Mango called "chandrakaran", am not sure about it though. Usually its made with mangoes which are smaller in size (at least I think so ;) ). My amma was saying sometimes a little jaggery is added towards the end of cooking to give it more sweetness. I haven't tried it that way. Please give it a try, if you like that flavor.