Cook cleaned vazhakka with salt, turmeric and 2.5 cups water. Bring it to a boil and let it simmer, till the vazhakka is cooked and water is almost dried.
Crush together coconut, green chillies, cumin and small onion (3), using a mortar and pestle or pulse it in the mixie for a few seconds.
Once vazhakka is cooked and water is almost dried, make a well in the centre and add the coconut mixture to this. Cover the coconut mixture with vazhakka and cover and cook for 5-7 mins on low flame. Remove the lid and mix well. If the mixture is sticking to the pan, add some hot water and mix well.
Heat oil in a pan and crackle the mustard seeds. Add curry leaves and small onion, when small onion begins to brown, add dry red chilli, fry for a min or two and add it to the vazhakka mixture.
Serve hot with rice.