In a bowl, mix the pork, scallions, ginger, mirin and salt. Gently fold in the crabmeat.
Dust a baking sheet with flour. Hold a gyoza wrapper in your palm, keeping the rest of the wrappers covered with plastic. Place about 1 1/2 tablespoons of the filling in the center of the wrapper and pinch the edges all around, forming an open cup. Transfer the dumpling to the baking sheet and cover. Repeat with the remaining wrappers and filling.
Fill a wok with 1 1/2 inches of water and bring to a boil. Arrange the cabbage leaves on 2 tiers of a bamboo steamer so they overlap slightly. Add the dumplings to the steamer and stack the tiers. Cover and steam until the shumai are just cooked, 8 minutes. Serve hot.