Cut the fish fillet into thick big peices. Toss them into the turmeric, chilli powder and salt. Set aside.
Keep the cornflour ready in a plate.
Heat a saucepan with oil. Dust each of the fish peices into the cornflour and place them into the oil. Once golden brown, flip the other side and cook till done. Drain the fish from the oil and set aside.
In the same oil, saute the ginger garlic paste till the raw smell is gone. Saute the onion till witled.
Add in the ketchup, chilli paste and water and allow it to boil till the sauce thickens.
Add a little sugar to remove the tanginess. Add back the fish peices and simmer for a couple of minutes.
Sprinkle in the garam masala, add salt and pepper to taste. Serve with coriander leaves to garnish.