Begin by washing & cleaning the crabs thoroughly.
Smear some turmeric powder & salt on the cleaned crab pieces & set aside.
In the meantime fry the potatoes in mustard oil over medium high heat. Fry them until nice & golden. Set aside.
In the same oil (add some more if needed) fry the crab pieces until the color changes to orange & you get a lovely aroma. Set aside when done.
Next, in the same kadhai or pan add some more mustard oil if needed & add the bay leaves + the dried red chilies. Allow them to sizzle for a few seconds. Do this over medium high heat.
Next add the sliced onions & saute until they become golden from the edges & limp but not crispy. Do not over fry them.
Now add the onion paste & the ginger garlic paste. Saute until the raw smell completely disappears and oil oozes out.
Next, add all the dry spices namely turmeric powder, cumin powder, red chili powder & the bengali garam masala. Give everything a good mix & saute unti nice, fragrant and oil leaves the sides.
It's time to add the pureed tomato & the tomato paste. Give everything a good mix & saute until oil oozes out.
Add the slit green chilies & mix well with the Bhuna masala or kosha moshla.
Next, add the fried crab pieces & mix everything very well so that the crab pieces are uniformly coated with the kosha moshla or bhuna masala.
Add hot or warm water, mix well.
Add the fried potatoes, mix well. Cover & cook over low flame until the potatoes are well done. Do the prick test (with a fork or toothpick) to check if they are done. It should take around 5-6 mins to get fully cooked.
Garnish with some more garam masala if you like the taste, mix well & serve hot with hot boiled rice. Enjoy.