Mix the egg, cornstarch blend, soy sauce, chopped green onions, and black pepper. Dip the crabs in the mixture to coat. Shake off the excess.
Heat the vegetable oil to almost smoking in a heavy skillet or wok over high heat. Fry the crab halves, two at a time, until crisp on the outside but still soft and moist inside. Remove from the pan, drain, and keep warm.
Pour out the oil from the pan, but don't wipe. Add all the other ingredients and stir to blend until boiling rapidly--about one minute.
Place one crab on each plate, and spoon the sauce over it. Serve immediately.