Slice the onion, tomato and green chillies lengthwise.
In a pan, heat oil and add chopped onions with curry leaves and green chilly. Cook for a couple of minutes.
When the onion starts to brown, add tomatoes followed by garlic and ginger.
Add chilly powder and garam masala. Cook for 4-5 mins. Keep it aside.
Grind together the coconut and cashews with a little water into a smooth paste.
Grind the onion-tomato mixture too into a fine paste.
Transfer back the onion-tomato mixture to the same pan. Add the crabs.
Cook on a low flame until the crabs are done.
Add the coconut- cashew paste to it and cook for 2 minutes.
Serve hot.