1. Wash and dry the prepared fish. Prick it on both side with a sharp fork or pointed knife.
2. Mix ingredients for 1st marinade and cover the fish with the marinade on all sides, as well as inside. Keep aside for a couple of hours. In hot climate, leave it in the fridge, covered.
3. To cook fish:
4. Grind to a paste in a blender onions, ginger, garlic, cashew nuts coriander, turmeric, chilli powder, salt and garam masala, all together. You will need to add a little water.
5. Heat oil in a pan and fry this mixture uncovered, stirring from time to time, until oil separates and the mix begins to come off the pan.
6. Add yoghurt and stir fry for few more minutes.
7. Add 1 cup water, saffron, lemon juice and cook some more. Taste and adjust seasoning. You need custard like consistency.
8. Add fish, bring to boil and simmer for around 8-10 minutes, until fish is cooked.
9. To garnish, heat oil in a ladle. Add mustard seeds and when they splutter, add curry leaves and broken up red chilli. Stir for 3-4 seconds and pour over the dish.

Serve hot with Rice.