Divide each crab into portions, by first removing the main shell. Next remove the two large claws and use a sharp knife to cut the body into 2 pieces, leaving the legs attached.
Place the onion, garlic, ginger, fenugreek, curry leaves, cinnamon, chilli, turmeric and coconut milk in a large saucepan. Season to taste, cover and simmer gently for 30 minutes.
Add the crabs to the simmering sauce and cook for 10 minutes to heat through. Serve immediately, with plenty of napkins.