Grind together coconut & turmeric powder with 1/2 cup water for 2-3 mins. Add 1/4 cup water to this and grind for 3-4 mins,till it becomes a smooth paste.
Make a tamarind paste by adding 1/3 cup water to it. Add green chilli, chilli powder, chopped ginger & salt to tamarind paste & mix well.
Take a Manchatti (clay pot) or a wide pan. Add coconut paste, tamarind paste and 1 cup water to this. Mix well. Bring it to boil.
Add cleaned fish pieces to this. Bring to boil and reduce flame to lowest. Cover and cook for 15 - 20mins or till the fish pieces are done and gravy (sauce) becomes thick.
Heat oil in a pan & splutter mustard seeds. Add sliced small / pearl onion & curry leaves. Stir till the onion becomes brown & pour this over the fish curry. Drizzle some coconut oil. Remove the curry from fire.
Notes
I used pomfret for this recipe. You can use any fish of your choice (it can be kingfish, mackerel, pomfret or even prawns).Let the curry rest for half an hour, before serving, for the flavours to settle in.This curry has a thick gravy (sauce). However you can add more water to adjust the consistency.