To marinate the fish: Mix all the ingredients for the marinade and evenly rub it into the fish fillets. Keep aside for 10 minutes.
To prepare the khaliya paste: Heat a tawa or a non stick frying pan, and separately roast the coconut and sesame seeds over very low heat until each emits its unique aroma (for about three minutes each). Grind the coconut, sesame seeds, groundnuts and onion with 3/4 cup water to a fine paste. Remove from the mixer. Add coriander powder and cummin powder. Mix well and keep aside.
Heat oil in a pan. Season with cummin seeds and curry leaves. Stir over medium heat until the cummin seeds begin to pop. Add ginger paste and garlic paste and stir-fry until the moisture evaporates. Add turmeric powder and chilli powder (dissolved in two tbsp water), and stir until the liquid has evaporated. Add the khaliya paste and stir-fry until the oil floats on top. Add tamarind pulp and stir-fry until the oil floats on top. Add the fish stock, stir well and bring to a boil. Lower the heat, sprinkle patthar phool powder and bay leaf powder. Stir and simmer till reduced to half its quantity. Add fish and bring to a boil. Lower the heat and simmer for about 10 minutes or till the fish is cooked. Add coriander leaves and green chilli pieces. Bring to a boil. Remove from heat and adjust the seasoning Serve with steamed rice or roomali roti.