1. Prepare the Filling: In a large bowl, combine ground pork, finely chopped napa cabbage, green onion, grated ginger, minced garlic, soy sauce, sake (or dry sherry), sesame oil, cornstarch, salt, and black pepper. Mix well until all ingredients are evenly distributed and the mixture becomes slightly sticky.

2. Assemble the Gyoza: Take one gyoza wrapper and place about 1 teaspoon of filling in the center. Lightly moisten half of the wrapper's edge with water. Fold the wrapper in half to create a crescent shape. Starting from one end, make small pleats along the moistened edge, pressing them firmly against the non-pleated edge to seal the dumpling securely. Repeat with the remaining wrappers and filling.

3. Prepare Dipping Sauce: In a small bowl, whisk together soy sauce, rice vinegar, sesame oil, and chili oil (if using). Set aside.

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

5. Steam Gyoza: Pour about 1/2 cup of water into the skillet (be careful, as it will splatter). Immediately cover the pan with a lid and reduce heat to medium. Let the gyoza steam for 5-7 minutes, or until the wrappers are translucent and the pork filling is cooked through.

6. Finish and Serve: Remove the lid and increase the heat to medium-high for 1-2 minutes to allow any remaining water to evaporate and further crisp up the bottoms. Carefully transfer the gyoza to a serving plate, crispy side up. Repeat with the remaining gyoza, adding more oil as needed. Serve hot with the prepared dipping sauce.