Clean, wash and devein the prawns. Sprinkle salt, turmeric and chilli powder over them, mix well and leave aside for 10 mins.
Meanwhile, chop the onions, slit the chilli, peel and cube the potato.
Heat a tablespoon of oil in a kadai. Toss in the prawns, lightly fry them for a minute or two and remove.
Add the panch phoron. Allow to splutter.
Put in the chopped onions and chilli and sauté for a minute.
Add the potato cubes, mix well, put in the prawns, add turmeric, chilli powder and salt, add some water, cover and cook on a medium flame till the prawns are done and the potatoes are nicely cooked. The dish should be almost dry.
Drizzle some mustard oil and remove.
Serve Chingri Macher Baati Chorchori with steaming hot rice.