Preheat the oven to 220°C/425°F/Gas 7. Place the (humanely killed) crab on a chopping board and lay a cloth on it. Using a hammer, or something similar, smash the body and the legs. Remove the large claws and crack them, but don’t break them open. As you finish each crab put it and any juices that ooze out into a roasting dish; when they’re all done place in the oven and cook for 30 minutes. When the 30 minutes are up take out the claws and add the chillies, onions, carrot, garlic and shrimp paste to the roasting dish. Cook for a further 20 minutes, then remove from the oven. Tip the contents into a large pot and cover with 3 litres (5 pints) of cold water; bring this to the boil, then turn down to a simmer. After 5 minutes skim off any scum, add the ginger, galangal, tomatoes, lime leaves and lemon grass and keep gently cooking for 2 hours. While it’s cooking remove the meat from the claws by cracking them open gently. Put the meat aside and add the shells to the pot.
To make the filling for the wontons, mix the prawn meat, grated ginger, ½ teaspoon fish sauce and spring onions together. Put in the fridge to chill for 30 minutes. Take a wonton wrapper and drop 1 small teaspoon of the filling into the middle. Brush around the filling with water, or egg wash, and fold the edges together to form a tight bundle. Repeat until the filling is used up. Store on a tray covered with non-stick baking parchment or clingfilm.
Now strain the stock well as it will contain many small pieces of shell. This should give you about 2.5 litres (4½ pints) of crab broth. Return it to the boil, then turn to a rapid simmer and add the bok choy. Drop the wontons in one by one and cook for 4 minutes from the time you add the last. While the wontons are cooking, shred the remaining stack of wrappers into 5mm (⅕ in) ‘noodles’ and drop these in, dispersing them as you go so they don’t stick together; cook for a further 30 seconds and turn off the heat. Test for seasoning and add more fish sauce and lemon juice to taste.
Divide the crab meat between the preheated bowls and scatter the coriander and mint over it. Then carefully ladle the soup, wontons, bok choy and noodles on top.