Roast coconut in a pan to brown colour and let it cool down.
Blend tomatoes, roasted coconut, garam masala, coriander powder, chilli powder, salt, garlic paste, tomato puree and turmeric powder to a fine paste.
In a pot braise onion in 3-4 tablespoon of oil till it's browned up.
Add coconut paste, chopped garlic and let it boil add little water and crabs, cook till done.
Simmer to make thick roast.
Garnish with chopped coriander leaves and green chillies.