Method

1. Soak the mustard and poppy seeds (if using) in 1/4 cup of warm water for 30 minutes. Then add all the ingredients listed under mustard paste in a food processor or mixer grinder and ground to a paste. If it feels too thick add a tbsp of water and grind it to a fine paste. Set it aside
2. Marinate the fish with 1/2 tsp turmeric powder and some salt and set it aside.
3. Heat oil in a thick bottomed pan on medium heat. When hot, temper it with nigella seeds and green chillies.
4. When the seeds crackles add the ground mustard paste along with turmeric powder, chilli powder, salt and sauté for 7-8 minutes stirring constantly.
5. Then add 1 to 1-1/2 cup of water. Check the seasoning and add sugar/sweetener and salt if required and bring it to a boil.
6. Now slowly slide in the marinated fish and cook for 7-8 minutes on medium heat. Simmer till the sauce thickens. There should be enough sauce to coat the fishes. It will not be thin and soupy.
7. Serve it hot along with rice.