Heat coconut oil in a pan.
Add onion, green chillies, ginger, garlic and curry leaves and fry, till they turn brown.
Add tomato and fry for 2 mins.
Add turmeric powder, coriander powder, chilly powder and salt and fry for 3 mins on a low flame.
Add tamarind water and mix it well.
Allow it to boil for 3 mins.
Add fish pieces.
Cover and allow it to cook on a low flame, till it becomes a thick gravy.
Add coconut milk.
Remove from the flame.
Heat a little oil in a small frying pan.
Fry small onions, till it becomes golden brown.
Add the above into the curry.