Step 1 In a medium bowl, lightly salt the chopped cabbage and toss to mix well, then set aside for 15 minutes. In batches, place the cabbage on a clean kitchen towel and wring the towel over the sink to squeeze out excess moisture. Set aside.
Step 2 In the bowl of a food processor, place the duck, green onions, one-fourth teaspoon salt, ginger, garlic, sake and miso. Pulse just until the mixture becomes a coarse paste. Transfer to a mixing bowl and fold in the reserved cabbage and the garlic chives, mixing well.
Step 3To assemble the gyoza, place a wonton skin in the palm of one hand. Spoon 2 teaspoons of the gyoza mixture onto the center of the wonton skin. With your finger, wet the wonton skin's edges with the cornstarch-water mixture. Fold the skin in half and pinch it together, pressing out the extra air before sealing the skin. (The cornstarch will "glue" the wonton skin together.)
Step 4Place the gyoza on a plate and repeat with the rest of the mixture, which will make about 25 pieces. Keep the wonton skins covered with a moist paper towel while you work, so they don't dry out. Set the gyoza aside.