First of all wash and clean the fish,marinate with little lemon juice, salt,turmeric and chilli powder, (I prefer to marinate fish even for the gravies)

Now grind all the ingredients to smooth paste, except oil, curry leaves and a onion

Take 2tbsp of oil in a wide mouthed pot, add mustard seeds, onion chopped,garlic chopped, green chillies slit and curry leaves, saute it for 2 to 3 minutes

Now add in ground masala paste saute it for five more minutes, add salt and 11/2 cups of water, bring gravy to boil, drop in marinated fish, let the fish cook on a low flame for 10 minutes, and it's done, serve hot with rice/roti.