If you haven’t purchased cleaned shrimp, clean and remove the heads. Leave the tails on, if you wish.
In a bowl large enough to hold the shrimp, mix the salt, 1/2 teaspoon of Kashmiri ground red chili pepper, turmeric, and baking soda (if using, see note 2). Add the shrimp and mix well to coat. Place in the refrigerator for 15 minutes, up to an hour.
Using a spice grinder, or mortar and pestle, grind the cashews (poppy seeds or almonds) to a fine powder. Strain through a sieve to remove any larger pieces to run through the grinder again and sieve.
In a large pan over medium-high heat, add 1 tablespoon of oil and heat the oil before adding the shrimp. Cook for 2 to 3 minutes (depending on the size of your shrimp), or just until they start to turn pink. Do not overcook because they will be simmered in the masala before serving. Remove from the pan and set aside.
Turn the heat down to medium and add another tablespoon of oil and the onion seeds or sesame seeds, if using. Let them crackle for a few seconds, then immediately add the onions. Cook for about 6 to 8 minutes, or until they have turned translucent and golden around the edges. Add a tablespoon or two of water if the onions begin to stick.
Add the minced green chilies, ground cashews (white poppy seeds or almonds), and 1/4 cup water. If you are not using mustard oil, add Dijon mustard now.
Sauté for 5 to 8 minutes to cook the spices, until the paste turns a golden brown and the oil begins to separate. IMPORTANT: If using cashew paste this step will only take 2 to 3 minutes!
Add 1/2 cup water, and sugar (if you used poppy seeds). Bring to a boil and add the shrimp and any juices that have been released. Stir gently but thoroughly.
Cover and simmer on very low heat for 5 minutes to allow the flavors to come together, the shrimp to finish cooking, and the sauce to thicken. This is a dry curry, with the sauce clinging to the shrimp, however, add additional water now, depending on how saucy you want your masala.
Finish by drizzling one teaspoon of mustard oil, if you wish. If you used Dijion mustard instead of mustard oil, your dish may need another bit of mustard and an additional sprinkle or two of salt. Serve with rice, a simple dal, or both.
Leave the tails on the shrimp for a nice look, if you prefer.
Using a bit of baking soda in the shrimp marinade helps give a snappier, more pleasant texture, but this is completely optional.
Chingri posto normally uses white poppy seeds to thicken the sauce. They are harder to find and I like the flavor of the sauce better with ground cashews. If you want to try cooking with poppy seeds, you must use the white variety; the more easily found blue variety tastes different and does not make a good substitution for the white ones. Besides cashews, another good option is skinless almonds.
This dish often includes fried cubes of potatoes, which I have omitted to streamline the recipe.
Bengali prawn curry is a dry curry. For my version, I’ve added slightly more water so that the dish has a bit of creamy sauce. You get to choose how saucy you want your shrimp masala.