Trim and finely slice the spring onions, remove and discard the tatty outer cabbage leaves, then shred very finely. Place in a bowl with the pork, sesame oil, rice wine or sherry and soy sauce.
Peel and finely grate in the ginger and garlic, season with a good pinch of white pepper, then mix well to combine. To check the seasoning, pinch off a little of the mixture and fry over a medium heat with a splash of oil. Taste and season again, if needed.
To make the potstickers, lay the gyoza wrappers on a clean work surface and cover with a damp tea towel to stop them from drying out. Lightly dust a tray with cornflour and place a small bowl of water alongside it.
Place 1 teaspoon of the filling onto the middle of a wrapper, brush the edges with a little water, then fold the wrapper in half over the filling into a half moon shape. Pinch and pleat the edges to seal, then place bottom-side down onto the flour-dusted tray. Repeat with the remaining ingredients – you should end up with 30 in total.
Heat a good splash of oil in a large non-stick frying pan over a high heat, then add the potstickers, bottom-side down, in a single layer (you’ll need to do this in batches). Reduce the heat to medium and fry for 2 minutes, or until the undersides are golden.
Pour 1cm of water into the pan, bring to the boil, then cover and reduce to a medium-low heat. Simmer for 6 to 8 minutes, or until the liquid has almost evaporated (if it reduces too quickly, top up with a splash more water halfway through).
Remove the lid and fry for a further minute, or until the undersides are crisp. Meanwhile, toast the sesame seeds in a small frying pan over a medium heat.
Combine the dipping sauce ingredients, then serve with the potstickers and toasted seeds scattered on top.