Roughly chop the cabbage, and finely chop the chives. Put both into a bowl. Add all the other ingredients except for the gyoza skins, knead and mix together.
Wrap the filling in the skins (be generous with the filling). Put a piece of plastic wrap on a work surface or dust with flour, and line the formed dumplings on top.
Heat a frying pan with vegetable oil. Line up the gyoza dumplings in the pan. (You don't have to brown them at this point). Once the dumplings are in the pan...
Add 100 ml of water (if you want to make "wings," dissolve 2 teaspoons of flour in the water first) to the pan, cover with a lid, lower the heat a bit and cook for 4 to 5 minutes.
When the dumplings have plumped up, the skins should be translucent enough so that you can see water inside the dumplings boiling away. Make sure that almost all the water in the pan has evaporated...
...then drizzle in vegetable or sesame oil, raise the heat a bit, and crisp up the bottom of the dumplings and the "wings." Lift up and peek at the edges to make sure the bottoms are browned. If they look crispy and golden, they're done.
To freeze them: Line a shallow container with plastic wrap and put the gyoza dumplings in a single layer on top. Put in the freezer. When the surface is hard and frozen, transfer to plastic bags and put back in the freezer. They are uncooked and don't last that long, so use them up quickly.
To defrost or thaw them: Just cook them as you would freshly made dumplings. Make sure the dumplings reach the stage described in Step 5. If the insides are still cold they will be awful.
If you are going to make the skins yourself, see Recipe ID: 202427 - "Ultra-Easy Gyoza Skins"
Story Behind this Recipe
I aimed for simple yet juicy gyoza dumplings. A Chinese friend of mine boils, chops and squeezes the Chinese cabbage, adds it to the filling, and then adds quite a lot of water too. So I wondered if you could just use the moisture in the vegetables and I started experimenting.
The key is to cook (or quick-freeze) the dumplings as soon as they're filled. The filling is rather watery, so if they leak, the dumplings burn easily. Be sure to secure the dumplings securely! This is a little trick - the ground pork and egg are very cold right out of the refrigerator and it's painful, so take them out at the same time you take out the vegetables to warm up a bit.