Add some salt and turmeric power to the cleaned fish and keep aside.
In a vessel or earthen pot take 2 cups of water.
To this add tomatoes, crushed onion, green chilies, curry leaves, chili power, coriander powder, turmeric powder, enough water and salt to taste.
Keep the prepared mixture on stove top and cook covered for 5 minutes.
Add the coconut paste and mix well.
Cook covered till the curry reduces a bit.
Add the fish pieces and mango pieces and cook covered till they are well cooked.
Heat 2 tbsp oil in a small kadai.
Add the finely chopped onion and fry till they are browned and nice aroma arises.
Pour this on to the curry and keep it closed. Serve hot with rice, idli or dosa.
Happy cooking :)