Using a large cleaver, cut the crabs in half and rinse well under
cold water, carefully removing the yellow gills or spongy parts.
Hit the legs and larger front nippers with the flat side of the
cleaver to crack the shell. This is to make the eating process easier.
Lightly and carefully coat the shells with a little flour.
Heat about 2 tablespoons cooking oil in a large wok and cook 1 1/2
crab at a time, carefully turning and holding the crab in the hot oil
until the shell just turns red.
Repeat with the remaining crab halves.
Add the remaining oil to the wok and cook the onion, ginger, garlic
and chili for 5 minutes over medium heat and stirring regularly.
Add the tomato sauce, water, soy sauce, chili sauce, vinegar and
sugar. Bring to boil and cook for 15 minutes.
Return the crab to the wok and simmer, turning carefully in the
sauce for 10 minutes or until the crab meat turns white.
Do not overcook.
Serve with steamed rice.
Note : It is important that you use only freshly caught crabs.