1.Freeze the crabs about 1 hour or until they are no longer moving. Place crabs upside down on a board. Lift the tail flap and insert the knife under the flap, straight down, all the way through the brain.
2.Remove the top shell and discard. Remove and discard the long feathery gills and shake out the grey or mustard-coloured substance. Clean by rinsing lightly and quickly.
3.Cut crabs in half lengthways, then cut each half into 3 pieces using a cleaver.
4.Crack the claws with a shellfish cracker, meat mallet or the back of a knife or cleaver.
5.Heat oil in a large wok. Stir-fry chilli, ginger and garlic until fragrant. Stir in sauces, stock and sugar; bring to the boil. Add the crab to the wok; toss until combined. Cover wok; simmer about 10 minutes or until the crab is just cooked through.
6.Stir in tomato and onion, stir until tomato has just softened. Season to taste with salt or extra fish sauce. Add coriander; toss to combine.