Clean and cut fish into pieces. Marinate with lemon juice, chilli powder pepper, salt and turmeric powder for 15-20 minutes.
In a kadai/pan, add little oil and shallow fry fish on both sides. Keep it aside.
In a separate pan, add oil and heat on a medium flame. Add the sliced onions and sauté until it turns translucent, ensure not to brown the onions.
Add curry leaves, crushed ginger garlic and green chilies, sauté for a minute until its raw smell subsides. To this you add chopped tomatoes and saute till it becomes soft.
Follow by adding turmeric powder, pepper powder and thin coconut milk. Close the lid and allow the curry to boil for 5-6 minutes on a medium flame until the oil starts separating.
Now add the fried fish, cover it with the gravy, and allow the curry to blend well with the fish, bring to a boil. Do not stir once the fish is added, instead rotate the pan to mix the gravy.
Add the thick coconut milk and cook on a low flame for another 2 minutes, make sure not to allow the dish to boil beyond this stage.
Garnish with more curry leaves and crushed pepper if you want it more spicy. Close the lid and switch off the flame.
Serve with Appam, Idiyappam, Chapati/Roti, Rice or Bread.