Here is a bit of advice. Don't ever come between a Bengali and his fish. In fact, just to be safe, don't ever come between a Bengali and his food. They take it very seriously. Afternoon naps are the only other thing that are as dear to a Bengali as his fish.
A Bengali meal is incomplete without some kind of fish curry in it. When I went back to visit my grandmother this past December, at some point I counted two types of fish curry, fried fish along with two other vegetable preparations on my plate. Yes, we take our fish very seriously indeed!
"Shorshe bata maach", or fish cooked in a mustard sauce is a quintessentially Bengali dish. It is a very simple dish as far as Indian curries go. There's no long list of spices to draw your attention away from what this dish is really about.
This dish works with any kind of firm fleshed fish. I have used snapper. Ideally, they should be cut in half inch thick steaks. Fillets work too, in a pinch.
In a cup of warm water, soak the poppy and mustard seeds overnight.
The next day, when you are ready to cook the fish, grind the soaked seeds along with the water. I used the small jar of the magic bullet.
Sprinkle half a teaspoon of turmeric powder and red chili powder along with some salt and rub it onto the fish. Leave it aside while you prep the rest of the ingredients.
Heat the canola and mustard oil in a pan and fry the fish in it in batches. Turn about half way through.
Once the fish is fried, take them out of the hot oil and set them aside.
Take out all but a tablespoon of oil from the pan. Add the nigella seeds in and wait for them to sputter. Add the remaining turmeric and red chili powder in.
Add the onion, ginger and garlic paste and fry it together for about 4-5 minutes till the onions are thoroughly cooked and it doesn't smell raw any more.
Add a cup and a half of water and stir it together.
Add the poppy- mustard paste and add a bit of salt and bring it to a boil.
Gently add the fish in and cook it in the mustard sauce A.K.A "jhol" for about a minute or two.
Add a couple of slit green chilies and serve.