In a large bowl, combine the ground pork, cabbage, nira chives, shiitake, garlic, ginger, soy sauce, sesame oil, sake, salt, and black pepper. Mix well with your hands.
Place a teaspoon of filling in the middle of a dumpling wrapper. Using your finger, lightly wet the half of the outer rim with water. Fold the wrapper in half. Using your fingertips, make pleats to seal the dumpling. Repeat with the remaining wrappers and filling.
In a large nonstick frying pan, heat the sesame oil over medium heat. Add the 20-22 dumplings in a circle. Fry for 1-3 minutes.
Combine the flour and the water in a small bowl or measuring cup. Pour into the pan and cover. Steam the dumplings until the water is mostly evapolated, 7-8 minutes. Remove the lid and continue cooking until the water is completely evaporated.
Place a plate on top of gyoza. Flip the pan upside down while pressing the plate to invert the dumplings. Cook the remaining dumplings.