Kill the crab just before steaming. Flip the crab over onto its back, and lift up the triangular piece of shell called the apron. Then stab the abdomen of the crab with a sharp knife.
Pour water into the base of a steamer and bring it to a boil. Arrange the crabs tightly in a single layer on the steamer’s insert and place it over the boiling water. Cover the steamer and cook for 20 minutes or until the crabs have turned completely red.
While the crabs are steaming combine all the dipping sauce ingredients in a saucepan and bring to a boil. Immediately turn the heat off and let cool.
Add the chopped scallion to the dipping sauce before serving. Serve the cooked crabs piping hot with the dipping sauce on the side.