Roast red chillies, coriander, cumin, mustard, methi; allow to cool. Grind the spices, coconut, garlic and tamerind with 1cup of water to smooth paste. Heat coconut oil and frythe sliced onion for 2 minutes; add grinded masala and 3cup water; bring to boil.
Reduce the heat and add the fish, green chilly, sliced tomato, chopped ginger and simmer for 8 - 10 minutes. Serve with boiled rice.