Wash the fish slices with salt and water. Strain the water out.
Heat a pan with soybean oil. Fry the fish pieces just slightly. Turn over the sides of the fish pieces for even frying. Transfer the fish pieces to a separate plate.
Add the onion slices to the same pan, and fry until the onions until slightly golden brown. Sprinkle turmeric, red chili and coriander powders. Also mix in the onion, ginger and garlic pastes. Pour in 1/2 cup of water. Mix and stir.
Cook until the water dries up a bit and the oil starts to simmer on top.
Place the fish pieces as a single layer in the spice mix. Sprinkle salt and stir carefully. Pour in 1 cup of water.
Put the lid on and cook on medium heat. Turn over the sides of the fish every now and then to ensure that the spices enter the fish well.
Cook until the water dries up a bit to form a thick gravy, and the oil starts to simmer or float on top.
Garnish with chopped coriander leaves and green chilies.