Take the marination ingredients and rub the fish pieces with turmeric powder and salt well. Keep aside for few minutes while you make the ingredients ready for the curry.
Heat mustard oil in a pan. Add the fish and shallow fry the fish pieces in batches for 3-5 minutes. Remove from the pan and keep aside.
In the same oil add cumin seeds, bay leaf, dry red chillies. Allow to splutter.
Add onion and saute until it turns soft and translucent.
Add ginger garlic paste and stir until the raw smell of ginger and garlic goes off completely.
Add the tomatoes and saute until it turns soft and mushy.
Add the turmeric powder, coriander powder, cumin powder, red chilli powder and salt as required. Stir on low-medium flame and cook for 1 minute.
Add warm water and give a quick mix. Cover and cook the mixture on medium flame for about 2-3 minutes.
Add the fish pieces and give a gentle mix. Cover the pan with lid and cook on low to medium flame for about 5 minutes.
Add the garam masala powder at this stage if using. Give a quick gentle mix, cover the pan and let it cook for 1 minute and turn off the flame.
Sprinkle some chopped coriander leaves and add the slit green chilly.
Your rui macher rosha is ready to be served now. Squeeze the lemon juice over the fish curry while serving.