Boil the cut(small pieces) washed cassava in water (fully immersed). Strain the water out.
Blend the other ingredients in a blender. Pour half of the blended mixture over half of the cooked cassava. Now add the other half of the cooked cassava and over it the other half of the blended mixture mentioned before. Simmer the cooked cassava with blended mixture for two minutes under light heat and under cover. Now mash the cassava with a wooden spoon. Set this aside.
Heat half a cup of coconut oil in a frying pan. When hot add a pinch of mustard seeds and let them break in oil. Add 50 grams of chopped onions and brown .Put some curry leaves. Now pour the hot oil with it's contents over the cassava previously prepared and mix .Serve hot with fish curry.