Thoroughly clean the crab with a stiff brush. Remove the top shell and the bottom flap. With kitchen shears, cut away the gills that line the sides of the crab. Cut the crab in half lengthways, remove the legs and quarter the body. Gently crack the claws with a cleaver (this will accelerate the cooking time and allow the aromatics to perfume the dish more, and make it easier to remove the meat later).
Choose a medium saucepan with a well-fitting lid. Put the sake, ginger and lemongrass in it and bring to the boil. Add the crab and cover. Turn the heat up high and once it comes to a boil, reduce the heat to a simmer and steam for about 10 minutes (cooking time will vary depending on the size of the crab and number used). Check to see that the sake does not completely evaporate. Remove the pan from the heat and leave it to sit, still covered, until completely cool-this will ensure that the crab remains moist and tasty.
Pick out the crabmeat. Put a quarter of it in a large mixing bowl and add the cabbages, cucumber, carrot, coriander, mint, pomelo, peanuts, half the sesame seeds and the sweet and sour sauce. Toss until well combined. Pile the salad on a serving platter and sprinkle with the remaining crabmeat and sesame seeds. Toss lightly. Drizzle with a little lemon oil to finish.