1.Combine cabbage, pork, onion, egg, sauce, sake, sesame oil, sugar and pepper in a medium bowl. Cover; refrigerate 1 hour.
2.Place one heaped teaspoon of pork mixture in the centre of a wrapper; wet edge around one half of wrapper. Pleat to seal. Repeat with remaining pork mixture and wrappers.
3.Cover base of a large frying pan with water; bring to the boil, then add gyozas, in batches. Reduce heat; simmer, covered, 3 minutes.
4.Heat vegetable oil in same cleaned pan; cook gyozas, one side only, uncovered, in batches, until browned and slightly crisp. Drain on absorbent paper.
Gyozas can be made ahead and frozen. No need to defrost before cooking; they will just take a little longer to cook.Note