Marinate fish with salt and turmeric powder and mix well. Set aside for 10 minutes. Meanwhile, strain the mustard paste.
Place a cooking pan on the gas stove and turn it on. Pour oil into it.
Once the oil become hot, reduce the flame, add black cumin, and stir.
Add two green chilies and stir again for a few seconds.
Add turmeric powder and red chili powder (½ tsp of each) to the pan.
Stir for a few seconds and add a cup of water.
Add the mustard paste which is made with mustard seeds, three green chilies, and a little bit of salt. Salt and green chilies overpower the bitter taste of black mustard. Stir for 5 minutes and add more water. Add salt.
Once the water starts boiling, gently add the fish to the cooking pan. Let it cook for 10 minutes.
Add more green chilies.
Add two teaspoons of mustard oil for a strong aromatic flavor.
Cover the cooking pan with a lid and let it cook for 5 minutes.
Turn off the gas stove. Pour the fish mustard curry onto a bowl. Serve it alongside steamed rice or shorshe bata diye alu (mustard-infused potatoes).