Prepare the vegetablesRemove the core of the cabbage and chop it finely. Finely chop the green onions and onion. Grate the garlic and ginger.
Make the fillingIn a large bowl, combine ground pork with the chopped vegetables, garlic, and ginger. Add cornstarch, toasted sesame oil, salt, black pepper, sake, and water. Mix very well with your hands or a spoon until the mixture is sticky and well blended—this stickiness helps the filling hold together.
Fill the wrappersPlace about 1 tablespoon of filling in the center of each gyoza wrapper. Use your fingertip to lightly wet the edges of the wrapper with water. This helps the wrapper seal tightly without tearing. Then fold in half, and pleat 4–6 times while sealing tightly so there are no gaps. Repeat with the remaining wrappers.
Cook the gyozaHeat a non-stick skillet over medium-high heat. (If not using non-stick, lightly oil the skillet before placing the gyozas to prevent sticking.) Arrange about 20–22 gyozas in the skillet without touching each other.
Steam the gyozaPour in ½ cup water and cover with a lid. Steam until about 90% of the water has evaporated (3–4 minutes). Remove the lid and let the remaining water evaporate.
Crisp the bottomsDrizzle a little oil into the skillet and continue cooking until the bottoms of the gyoza are crispy and golden brown.
ServeServe hot with gyoza dipping sauce.