Prepare the Filling: In a large bowl, combine ground pork, finely chopped napa cabbage, minced shiitake mushrooms, chopped green onions, grated ginger, minced garlic, soy sauce, sesame oil, sake (if using), cornstarch, and white pepper. Mix until just combined, do not overmix, to keep the texture light.

Fold the Dumplings: Place a gyoza wrapper in the palm of your hand. Scoop about one teaspoon of filling into the center. Lightly moisten half of the wrapper's edge with water. Fold the wrapper in half to create a half-moon shape. Starting from one end, make 3-4 small pleats on the top layer of the wrapper, pressing them against the unpleated bottom layer to seal securely. Repeat with remaining wrappers and filling, keeping finished dumplings covered with a damp cloth to prevent drying.

Pan-Fry the Gyoza: Heat 1 tablespoon of vegetable oil in a large non-stick skillet or cast iron pan over medium-high heat. Arrange the gyoza in a single layer, ensuring they don't touch. Pan-fry for 2-3 minutes until the bottoms are golden brown and crispy.

Steam and Finish: Pour 1/4 cup of water into the skillet (be careful as it will sizzle). Immediately cover with a lid and steam for 5-7 minutes, or until the pork filling is cooked through and the water has evaporated. Remove the lid and continue to cook for another 1-2 minutes, if needed, to ensure the bottoms are perfectly crispy again. Repeat with remaining gyoza, adding more oil as needed for subsequent batches.

Prepare Dipping Sauce: While gyoza are cooking, combine soy sauce, rice vinegar, and chili oil (if using) in a small bowl, stirring well.

Serve: Serve hot immediately with the prepared dipping sauce for an authentic experience. Enjoy your homemade crispy gyoza!