Bring a pan of water, large enough to hold the 4 crabs with plenty of room, to the boil. Season the water with the salt, peppercorns, bay leaves and lemon. Add the crabs–upside down so the claws will not break off–and bring the water back to the boil. Boil for 5 minutes, turn off the heat and leave the crabs in the water for 15 minutes, then plunge them into cold water, drain, and leave to get cold.
Remove all the white meat from the body, leaving the brown meat inside the shell. Be careful to discard the possibly poisonous dead men’s fingers. These are gills which filter water, the purity of which cannot be guaranteed. Return the white meat to the body shell and crack the claw shells to expose the meat.
Make the dressing with the rice wine vinegar, dark soy sauce, ginger, garlic, sugar and 2 finely chopped spring onions. Mix the sauce with a fork to amalgamate well. Spoon some of the sauce over the crab body and claws. Serve with 2 spring onions per person.