Make Gyoza Dough: In a bowl, mix medium protein wheat flour and salt. Slowly pour boiling water while stirring with chopsticks or spatula. When cool enough, knead with hands until no flour granules remain. Rest dough for at least 2 hours.
Shape Gyoza Skin: Roll dough into a tube and cut into pieces. Dust with cornstarch to prevent sticking. Flatten each piece with a rolling pin into a round shape about 8 cm diameter. Optionally use a cutter or glass to shape.
Prepare Filling: Chop chives (or spring onions) and boil cabbage for 2-3 minutes, then chop. In a bowl, combine ground chicken thigh, grated garlic and ginger, salt, sugar, black pepper, soy sauce, oyster sauce, sesame oil, cabbage, and chives. Add water and oil, mix well until slightly wet but scoopable.
Form Gyoza: Stretch skin slightly longer than round. Place about 12-15 grams (1 tablespoon) of filling on skin. Moisten edges with water, fold in half, and pleat edges 5-6 times to seal tightly.
Prepare for Freezing: Dust bottom of each gyoza with cornstarch to prevent sticking and create crispy bottom when cooked. Arrange gyoza in a single layer on plastic-lined tray and freeze. Stack only after frozen.
Cook Gyoza: Heat pan until hot, add oil, arrange gyoza bottom side down. Add water and cover pan. Cook on low heat until water evaporates and bottoms turn crispy brown. Cooking time approximately 6-7 minutes.
Make Dipping Sauce: Heat oil and sesame oil, pour over chili powder (Gochu garu or togarashi) to make chili oil. In a bowl, mix soy sauce, black vinegar, sugar until sugar dissolves. Add toasted sesame seeds and stir. Adjust sweetness or sourness with water if needed.
Serve: Serve cooked gyoza hot with the prepared dipping sauce and optional chili oil.