1. Rub the fish on both sides with a generous sprinkling of salt, lots of freshly ground pepper and the turmeric and chilli powder. Rub well and set aside.
2. Put 3 tablespoons oil in a non-stick frying pan and set over high heat. When hot, put in the fish and brown quickly on both sides without letting it cook through. Remove with a slotted spoon and set aside. Add the onion and garlic to the pan. Stir and fry until the onion browns a bit. Add the green coriander, tomato, green chillies, ginger, lemon juice and salt. Stir and cook over medium–high heat until the coriander and tomato have completely wilted. Spread the green sauce around in the frying pan and check the salt.
3. Lay the fish over the top. Now spoon some green sauce over the fish. Sprinkle the chilli powder and garam masala over the top. Bring to a simmer. Cover, lower the heat and simmer gently for about 10 minutes & Serve.