Heat oil in medium high heat. Put curry leaves and stir. Add ginger and garlic and sauté. When it is light brown, add sliced onion and sauté till golden. Reduce heat to medium. Add chili powder, coriander powder, turmeric powder and fenugreek powder and roast well. Add the tamarind mixture, tomatoes and mix well. Raise heat to medium high. Add the water and bring to boil. Add salt, mix well. Add the crab pieces, mix.
Cook covered, mixing occasionally. Cook until done, remove cover and cook until the gravy is thick.