For the Bhetki shorshe bata, wash the fish pieces well and pat dry. Marinate the fish pieces with salt, turmeric and chilli powder and keep aside for 10 minutes.
In a small bowl take a little mustard powder, add a pinch of salt and 2 tbsp of water and make a smooth paste. Set aside for about 10 minutes.
Finely chop the tomato (optional) and coriander leaves. Slit the green chilli.
Heat oil in a frying pan, arrange the marinated fish pieces on the pan and lightly fry them. Remove onto a plate and keep aside.
Add some more oil to the frying pan. Splutter kala jeera and green chilli.
Add the finely chopped tomato (optional) and cook till they turn mushy.
Sprinkle haldi and chilli powder and stir well.
Mix in the mustard paste.
Add a dash of salt, 2 cups of water and bring the mustard curry a boil. Lower the heat, cover with a lid and simmer for about 5 minutes on medium heat.
Gently slide the pan fried fish into the mustard gravy. Cover and cook for 10 minutes till the fish is done and the gravy thickens.
Pour a little mustard oil on the fish curry and mix it in gently.
Garnish with fresh coriander leaves and remove mustard fish curry on to a serving bowl.
Serve Shorshe Bata Diye Maacher Jhal, with steamed rice and dal.