To prepare the cabbage and Chinese chives: Bring a large pot of water to a boil over high heat. Boil the cabbage leaves for about 3 minutes, or until softened, then transfer them to cold water. Blanch the Chinese chives for about 30 seconds, then transfer them to cold water. Reserve 1 cup of the vegetable cooking liquid, then drain the cabbage and chives well.
To make the filling: In a large mixing bowl, gradually pour the cooled reserved vegetable cooking liquid into the ground pork, mixing until fully absorbed. Add the soy sauce, oyster sauce, white pepper, sesame oil, and egg, and mix well. Fold in the scallion and Chinese chives until evenly combined.
To assemble the dumplings: Lay one large cabbage leaf flat and top it with a smaller cabbage leaf if needed. Place about 1 tablespoon of filling in the center. Gather the cabbage around the filling to form a bundle, then tie it securely with a blanched Chinese chive. Trim any excess chive. Repeat with the remaining ingredients.
To steam the dumplings: Bring a steamer to a boil over high heat. Arrange the cabbage dumplings on a heatproof plate and place it in the steamer. Cover and steam for about 10 minutes, or until the pork reaches an internal temperature of 160°F (71°C).
To make the sauce: Pour the juices collected on the steaming plate into a small saucepan over medium heat. Bring to a simmer, season with a pinch of salt, then stir in the cornstarch slurry (1 teaspoon cornstarch mixed with 1 tablespoon water). Cook until the sauce is slightly thickened and glossy.
To finish and serve: Spoon the sauce over the cabbage soup dumplings and serve immediately.