Wash the fish pieces and set aside for the water to dry.
Heat the vegetable oil in a pan and fry the garlic and onion slices. When the onions and garlic turn golden brown, add the cumin and the ginger paste. Sprinkle the salt, red chilli and turmeric powders. Add 1/2 cup of water and stir well so that the spices do not burn.
Cook until the water dries up and oil starts to float. When the oil starts to float, add 1 cup of water. Repeat 2 more times. This process of drying the water, allowing the oil to float on top and then adding water is called “koshano” in Bengali.
Now add 1.5 cups of water and add the fish pieces. Put the lid on when bubbles begin to form. Lower the heat to medium level and check every now and then to see if the fish pieces have been cooked.
Cook for 10-15 minutes until water dries to form a gravy and oil starts to float on top. Garnish by sprinkling some coriander leaves.