Clean, wash and finely chop fresh coriander leaves. Soak tamarind in one cup warm water for half an hour, remove the pulp, strain and keep aside. Heat a tbsp of oil in a pan and fry sliced onion, sliced ginger, scraped cocunut and garlic, till light golden brown. Remove from heat and cool. Add a little water and grind into a thick paste.
Heat remaining oil in a pan, add mustard seeds, when they start to crackle add curry leaves.
Immediately add masala paste and cook on medium heat for half a minute, stirring continuosly. Add turmeric powder, malvan masala, chilli powder, tamarind pulp and salt. Stir well and add 1 cup of water. Bring it to a boil and add crab pieces and claws. Cook further on medium heat for 10-15 mins,stirring occasionally or till the gravy becomes a little thick.