Fish (Cod) in mustard sauce

Ingredients

500 grams Cod fillets (boneless)
1/2 teaspoon salt
1/2 teaspoon turmeric powder
1/2 teaspoon chilli powder
1 tablespoon mustard seeds
1 big onion - roughly chopped
1 tomato - roughly chopped
4 green chillis - roughly chopped
1 inch ginger - roughly chopped
1/2 teaspoon ajwain /carom seeds
1/2 teaspoon chilli powder
1/2 teaspoon turmeric powder
1 teaspoon dhania jeera powder
salt to taste
2 tablespoons mustard oil (I used vegetable oil)

Method

Cut the cod fillets into 1 inch pieces. Marinate in the next three ingredients and let it stand for 15 minutes. In a frying pan, add 1/2 tablespoon oil and fry the cod pieces till you get a nice sear on the outside. Remove and let it sit in a plate.
In the same frying pan, add the rest of the oil. When it gets hot, add the mustard seeds and let it splutter. Then add the onions and fry till they turn a nice golden in color. To this add the ajwain, tomoato, green chillis and ginger and saute for about 10 to 15 minutes till the tomatoes get all mushed up. Add the chilli powder, turmeric powder, dhania jeera powder and salt and cook for about 2 to 3 more minutes. Let this mixture cool. Once the mixture is cooled, bled it to a paste in a grinder/blender. Add it to the frying pan along with 1/2 cup water and let the mixture boil. Be careful to keep the pan covered as this mixture will splutter. After about 5 minutes, lower the heat and add the seared fish pieces. Mix carefully and cover and let the mixture simmer on medium heat for another 5 to 10 minutes.
Serve hot with phulkas / lucchis (puris)/ Dal & Bhaat (Rice)

I do not know if this is the most authentic way of making the famous Bengali fish in mustard sauce, but this was my quick version. 
Moreover, Bengali's, use Rohu/Hilsa or other river fish to make their curries. I had cod in my fridge and hence used that.