Cut and clean the fish. Make medium sized slices. Then make slits on the flesh of the fish.
Mix the fish with lemon juice, turmeric powder, ginger paste, 4 tablespoons soybean oil, yoghurt, onion paste, red chilli powder, garlic paste, salt, cumin and coriander powders.
Allow to marinate for 2 hours. In between, reverse the sides of the fish to allow the ingredients to enter the fish well.
Heat a big frying pan. Add the fish slices along with the ingredients used for marinating. Fry on low heat. Every now and then reverse the sides and with a spoon cover the fish with the marinate sauce. When the fish slices have been fried, set aside the fried fish.
In a separate pan, heat 1/2 cup of soybean oil. Add onion slices and fry until crispy golden brown (peyaj beresta). Mix garam masala powder and chopped green chillies when the onions are fried. Pour this mixture all over on the pan with the fried fish slices.
Turn the heat on and cook the fish again on very low heat for 8-10 minutes.