Soak the raw rice with few 4 tblsp of water for atleast 15 minutes.
Meanwhile, heat a kadhai or a thick bottom steel vessel with 2 cups of water and add the cubed pieces of ash gourd along with turmeric powder and allow it to cook.
Ensure the ash gourd is just cooked right else it will turn mushy. Prick it with a fork and it should be firm and shouldn’t squash.
Now prepare the yoghurt mixture by whisking the yoghurt with some water and keep aside. The mixture should be of lassi consistency.
In a mixer grinder jar, add the grated coconut, soaked raw rice along with the water, green chillies and grind it into a thick paste, using some water, if required.
Add the gravy paste to the cooked ash gourd. add salt and mix well. The gravy would thicken due to the rice added.
Now add the yogurt or buttermilk. Mix well and immediately switch off the flame.
If you continue to cook this on high flame, the buttermilk would split.
Prepare for tempering by heating some coconut oil in a pan, add mustard seeds, once they crackle add the whole red chillies and curry leaves.
Pour the tempering over the hot gravy, mix and serve with some steamed rice, poriyal or vegetable of your choice.