Mix water with flour with a spatula until it's cold enough to be kneaded by hand.Knead for about 10 minutes and wrap in cling film.
For the filling:
Chop finely cabbage and chives, add grated ginger and garlic, mix everything with minced pork.Adjust salt and pepper, add soy sauce, sake and sesame oil.Mix everything well.
Make pieces of dough of about 10-12 g each, spread them round shaped helping yourself with some potato starch if the dough is sticky.Put a spoonful of filling on the dough disk and close making pleats ( how to make them is far easier to see than to explain, you can watch the video I added in the post above).
Put the dumplings into a pan greas with oil.Let the bottom fry for a minute then add some water ( enough to reach half the height of the gyoza), cover with a lid and let it simmer until gyoza are done and all the water is evaporated.If the water evaporate before gyoza are cooked add some, little by little.
When ready remove the lid, drizzle with some sesame oil, let the bottom fry a little more and be sure all the water is evaporated.Serve hot.
Mix all the ingredients to make the sauce; if you don't have all of them you can dip them in plain soy sauce.
With this recipe you can make about 40 gyoza. You can freeze them before cooking to have them ready whenever you want.Do not defreeze them before cooking, just extend the cooking time.