Place mud crabs in freezer for 30 minutes or until they have gone to sleep. Turn crab upside down and lift tail (or flap). Pull this away from the crab's belly, turning crab over as you do so and pulling away main shell of crab as well. Pull away long grey 'dead man's fingers' and any other little bits that are not the claws; discard. Wipe crab clean but try not to immerse in water.
Place crab on a chopping board and, using a heavy bladed knife, cut crab in half through body. Cut off main claw and then cut remaining part of body into 2 halves. Using back of the knife, bash shell of claws to allow flavour to get to meat and crab to cook more easily. Repeat process with second crab and set crab pieces aside.
Combine salt and both types of peppercorns in a mortar and pestle and pound until they are finely crushed.
Heat oil in wok and add ginger, garlic and half of the salt and pepper mixture. Stir-fry for 1-2 minutes or until mixture is very fragrant. Add crab and stir-fry until almost cooked. Finally, add combined shao hsing wine and sugar and bring to the boil. Cover and simmer for 5 minutes.
Toss in green onions and half of the coriander.
Serve immediately with remaining salt and pepper mixture for guests to help themselves.
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