Marinate the curry cut rohu fish with turmeric and salt. Heat a wok with 6 tbsp mustard oil and fry the rohu in medium-high flame. Place the fried pieces in a paper towel and keep them aside.
To make the curry, you will need one bunch of spinach, two medium-size potatoes, one large tomato, one-inch ginger, one green/red chilli, one tsp each turmeric and black pepper powder, salt and water as required.
Finely chop the spinach, finely wedge cut the potatoes, dice the tomato, crush the ginger and slit the chilli.
Cook the curry in the same wok used to fry the fish in the remaining oil. Put all the ingredients together and let it steam on low flame for 6-7 minutes.
As the water from the spinach recedes, add turmeric and black pepper powder, mix it properly, add 1 cup water with salt to taste and let it simmer for another 6-7 minutes in medium-low heat to finish the dish.
To serve, place the fried rohu first and pour the curry. The dish tastes best with steamed rice.