Clean the crab by removing the shell and chopping into pieces; crack the claws and legs by hitting with the blunt edge of a heavy cleaver.
In a wok, heat vegetable oil over a low heat. When hot, fry garlic until aromatic and golden. Add the crab and stir fry for 3-5 minutes or until cooked through. Mix seasoning sauce ingredients with chicken stock. Add to the wok and bring to a boil, stirring. Boil for 3-5 minutes.
Add egg to the wok and stir well until cooked. Add milk, tapioca flour, Chinese celery, red chili. Stir well until cooked. Spoon onto a serving plate.