To prepare the cabbage and chives, bring water to a boil in a pot over high heat. Add the cabbage leaves and blanch for about 2 minutes until softened, then transfer to cold water. Blanch the Chinese chives for about 30 seconds, then transfer them to cold water. Drain and reserve 1 cup of the vegetable boiling liquid.
To make the filling, in a bowl, add the ground pork. Slowly pour in the reserved vegetable liquid while stirring so the meat absorbs the liquid. Add oyster sauce, white pepper, sesame oil, and egg, and mix thoroughly. Stir in the shiitake mushrooms, carrots, and scallions.
To assemble the rolls, place a cabbage leaf flat and add about 2 tablespoons of filling in the center. Fold the bottom over the filling, fold the sides inward, and roll tightly. Tie with a blanched chive and place seam-side down on a plate.
To steam the cabbage rolls, in a steamer over high heat, bring water to a boil. Place the plate of cabbage rolls inside, cover, and steam for about 15 minutes until cooked through.
To make the sauce and serve, pour the liquid collected on the plate into a small pan over medium heat. Bring to a simmer, season with a pinch of salt, and stir in a slurry made from cornstarch and water. Cook until slightly thickened, then drizzle the sauce over the cabbage rolls.