I would never serve shell-on crab at a formal dinner party, or order it at a restaurant on a first (or even second) date. Because they are so messy to eat, crabs should be reserved for casual meals with good friends.
The flower crab is named for the pattern on its shell. For this dish, other types of small crab (weighing about 250g/9oz each) can be substituted. Choose lively specimens that feel heavy for their size.
Take the crabs from the bag you brought them home in and lay them on a tray. Place the tray in the freezer and leave them for 10 minutes - this puts the crabs to sleep. (Don't leave them in the freezer for longer than 10 minutes, or they will be frozen.) Kill the crabs by quickly pushing the sharp, slender blade of a boning knife, or a sharp skewer, through the face into the brain.
Clean the crabs by scrubbing them with a rough brush under running water. Flip them onto their backs and pull off the loose flap of shell at the base - on males the flap is elongated; on females, it is a wide bell shape. Turn the crabs over again. Firmly hold down the claws in your non-dominant hand (you might want to put a kitchen cloth under your hand); with your other hand, grasp the side of the top shell and pull it away from the body. Place the top shell cup side-up on a work surface. From the body, pull out and discard the feathery gills, the intestines (they're white and squiggly) and other inedible parts. Use kitchen shears to trim off the eyes and mouth parts, then briefly rinse the body under cold water. Cut each crab body into four pieces, making sure some of the legs are attached to each quarter. Leave the body pieces and top shells at room temperature while preparing the rest of the ingredients.
Cut the ginger into thick slices, then bruise them with the flat side of a cleaver. Roughly chop the garlic and slice the onion about 3mm (⅛in) thick. Cut the chillies on the diagonal into pieces about 5mm (⅛in) thick. Lightly bruise the lemongrass with the flat side of a cleaver, then slice into thin rounds. Chop the spring onion into 2.5cm (1in) lengths.
Place a large wok over a high flame then add the oil. When the oil is hot, add the ginger, garlic, onion and chilli and stir-fry for about 30 seconds.
Stir in the lemongrass, then add the crab body parts and top shells. Add the fish sauce and sprinkle with salt and pepper. Stir the ingredients well, then cover the wok with the lid, turn the flame to medium and cook for about two minutes, mixing occasionally. Stir in the coconut milk and spring onion and bring to the boil over a high flame. Turn the flame to medium then cover with the lid and simmer for about five minutes or until done, stirring often. The crabs are ready when the shells darken in colour.
Taste the sauce and adjust the seasonings, if necessary. Scoop everything onto a dish and serve immediately. Serve with steamed rice and stir-fried green vegetables.