In a large bowl, mix the pork mince, shredded Chinese cabbage, spring onions, grated ginger and very finely chopped garlic. Stir in the soy sauce, sesame oil, cornflour, salt and pepper, then use your hands to combine everything until the mixture is well blended and slightly sticky.
Place a gyoza wrapper in the palm of your hand and lightly moisten the edge with water. Spoon about a teaspoon of filling into the centre, fold the wrapper in half to form a half-moon and pleat the edges to seal. Repeat with the remaining wrappers.
Heat the vegetable oil in a non-stick pan over a medium-high heat. Arrange the gyoza in a single layer and fry for 2-3 minutes until the bases are golden and crisp.
Add 100 ml water to the pan, cover with a lid and steam over a medium heat for 5-7 minutes, until the water has evaporated and the wrappers are tender. For crispier bases, remove the lid and increase the heat for a further minute.
Whisk together the soy sauce, rice vinegar and, if you like, a few drops of sesame oil and chilli oil in a small bowl. Serve the gyoza immediately with the dipping sauce.