For cleaning the crabs: Wash the crab well. Ensure that all dirt from the shell is removed. Wash it with a bristle scrubber if required.
Remove legs and claws and break each into two and crack the bigger pieces. This will help the spices to get to the meat easily. Make slashes or poke holes. Clean the meat well by removing all the yellow/brown gooey stuff. Remove the top shell and discard. Clean the meat that is in the bottom shell. Cut each into four pieces.
For preparing the dish, first heat oil in a pan on medium high heat.
Put in curry leaves and stir.
Add ginger and garlic and saute.
When it is light brown, add sliced onion and saute till golden.
Reduce heat to medium and add chilly powder, coriander powder, turmeric powder and fenugreek powder and roast well. Add the tamarind mixture and mix well.
Raise heat to medium high and add the water and bring to a boil. Add salt and mix well.
Add the crab pieces and mix well.
Cook covered, mixing occasionally. Cook for about 15 minutes.
Remove cover and cook until the gravy is thick enough to coat the crab.
:- For best results, cook at night and reheat and reduce the gravy the next day. Do not refrigerate during the night. This way, the meat will absorb the spices well.