First, gently clean the fish pieces and rub them with a little salt and turmeric powder. Let them sit for about 10 minutes.
Next, grind mustard seeds into a smooth paste using a grinder or a traditional grinding stone- Shil Nora / Shil Batta. Mix a pinch of salt into the mustard paste and set it aside for approximately 15 minutes.
Make a poppy seed paste by grinding poppy seeds or Posto dana with a little water and salt. This will add a wonderful richness and texture to the sauce. Adding Posto is optional though.
Now, blend the tomatoes into a paste. For a more rustic flavor, you can also chop the tomatoes instead.
Heat 2 tablespoons of mustard oil in a wok until it's nice and hot. I prefer using Iron Kadhai for this recipe.
Carefully fry the fish pieces in the oil over medium heat until they turn a beautiful golden brown from both side. It will take 3-4 minutes per batch or 2-3 fishes to fry them properly.
Remove the fish from the oil and set aside.
In the remaining oil, add nigella seeds (kalojira) and sliced green chilies. Let them sizzle for a moment to release their flavors.
Pour in the tomato puree or chopped tomatoes and cook for about 5 minutes, or until the raw tomato smell disappears.
Add salt and turmeric powder to the sauce.
For a vibrant color, you can also add a touch of Kashmiri red chili powder.
Continue cooking until the oil separates from the edges of the sauce.
For a stronger mustard flavor, let the paste sit for a few minutes. Mix 3 cups of water with the mustard paste, then strain the liquid to remove the mustard skins. Pour this mustard water into the gravy and bring it to a boil.
Gently add the fried fish pieces to the gravy.
Cook over medium heat for 8-10 minutes, carefully turning the fish to ensure it's well coated with the delicious mustard gravy.
Adjust the gravy's thickness to your liking; I like it medium-thick.
Stir in 1 tablespoon of raw mustard oil and cook for an additional 2 minutes.
Turn off the heat and serve this flavorful Mustard Fish Curry (Shorshe Bata diye Macher Jhal) with plain rice.