To pork add 1 minced garlic clove, a few tablespoons of water, a few splashes of soy sauce, 1/2 teaspoon sugar, and Chinese mushrooms.
Clean crabs, drain and plunge into a large pot of boiling water for 30 seconds. Remove, drain and scrub. Snap off claws, crack them and reserve. Twist off apron, separate shell from body and remove gills, small legs and stomach sac. Quarter bodies, leaving roe and tomalley intact.
Heat oil in a wok. Add ginger and garlic, half of scallion and drained black beans. Push aside and add pork mixture. Stir-fry, breaking up pork and mixing well with other ingredients. Push aside, add crabs and a few splashes of sherry. Mix together remaining 1/2 teaspoon sugar, thin soy sauce, stock and cornstarch. Add to wok and stir-fry until sauce thickens. Remove from heat. Add eggs and sesame oil. Transfer to a serving platter. Garnish with remaining scallion and serve immediately.