In a large bowl, combine the ground pork, shredded cabbage, green onions, garlic, ginger, soy sauce, sesame oil, mirin, sugar, white pepper, and salt. Mix until combined.
To assemble each gyoza, lay a wrapper on a plate and place a tablespoon of the meat mixture in the middle, leaving a little bit of space around the edges.
Wet the edge of the wrapper with a little bit of water, fold the wrapper in half, and press the edges together to seal.
Using your thumb and forefinger, pinch and fold one end together to create a small pleat, move to the next section of the wrapper, and repeat the pinching and folding process - ensuring you seal the folds well.
Continue working your way around the wrapper, pinching and folding to create a series of pleats until you reach the other end.
Repeat with the remaining wrappers and filling. Place them on a plate or baking sheet until you are ready to cook.
When ready to cook, heat a large non-stick frying pan over medium heat and add enough vegetable oil to coat the bottom of the pan.
Once the oil is hot, add 6-7 gyoza to the pan, flat-side-down, and cook for 2-3 minutes or until the bottom turns golden brown.
Add ¼ cup of water to the pan, cover with a lid, and steam for 2-3 minutes or until the gyoza are cooked through and most of the water has evaporated.
Then, remove the lid to evaporate any remaining water and continue to cook uncovered until they are golden brown and crispy on the bottom.
Carefully remove them from the pan to a clean plate. Repeat with the remaining gyoza.
Serve hot with gyoza sauce and garnished with green onions and sesame seeds if desired.