Prepare the crab by by hitting the back underside of the shell, which loosens the meat. With the shell towards you, stand the crab up and force the body away from the shell using your thumbs. Pull off the tail and discard it, then twist off the claws and smash them open by hitting them with a rolling pin. Pull away the gills that lie along the body and discard them. Using a sharp knife, split the body into quarters.
Heat the oil in a wok over a high heat until the oil starts to shimmer. Add the chilli, lemon grass, garlic and ginger and stir-fry for 30 seconds. Pour in the soy, fish and sweet chilli sauces and water.
Add the crab and stir-fry for 2–3 minutes until well coated in the sauce and warmed through, then remove from the heat. Stir in the lime juice and fresh herbs.