For the dumplings: Pulse the mortadella in a food processor until finely chopped. Place the mortadella and ground pork in a large mixing bowl. Add the cornstarch, mirin, sesame oil, shaoxing wine and soy sauce, and sprinkle with salt and pepper. Mix and incorporate well by hand. Add the cabbage, onions, ginger, garlic and scallions to the pork mixture and knead well by hand. Cover the bowl and refrigerate for 20 to 30 minutes to let the flavors meld.
Fill a small bowl with warm water. To form dumplings, place a tablespoon of the filling inside each dumpling wrapper. Wet the outer edge with the warm water and seal tightly by forming pleats along one side. Continue until all the filling is used and formed into dumplings.
Heat 1 tablespoon of sesame oil in a nonstick skillet over medium-high heat and place some dumplings in the skillet without crowding. After 30 seconds, add hot water to the skillet to cover the dumplings about a quarter of the way up. Cover the skillet, turn the heat down to medium and let the dumplings steam. Remove the lid after the water is mostly evaporated, 5 to 6 minutes. When the skillet is dry, add an additional tablespoon of sesame oil, turn the heat back to medium-high and let the bottom of the dumplings get crispy and brown. Repeat with remaining dumplings.
For the dipping sauce: Mix together the soy sauce, balsamic vinegar and chili flakes. Serve with the dumplings.