Drain the mustard seeds and add them to a mixer grinder along with all the other ingredients (except the oil) and blend to a smooth paste
In a mixing bowl, mix this mustard-coconut paste with the prawns well and set this aside for 15-20 minutes
Heat your cooking pot with the mustard oil until it smokes, then lower the flame and add the nigella seeds. Pour in the marinade along with the prawns
Wash your mixing bowl with ¼ cup water and add to your cooking pot (so that you don't waste any of the marinade). Cover and cook over a medium flame for 15-17 minutes
Remove from flame and drizzle over a tablespoon of mustard oil and serve your delicious sorshe narkel chingri (mustard coconut prawn curry) with steamed rice and a lime wedge on the side. Enjoy!