To prepare dumplings, combine first 10 ingredients. Working with 1 gyoza skin at a time (cover remaining skins to prevent drying), spoon about 1 1/2 teaspoons pork mixture into center of each skin. Moisten edges of skin with water. Fold in half, pinching edges together to seal. Place dumplings, seam sides up, on a baking sheet sprinkled with 1 teaspoon cornstarch (cover loosely with a towel to prevent drying).
Arrange half of the dumplings in a single layer in a bamboo or vegetable steamer coated with cooking spray. Steam dumplings, covered, for 10 minutes. Remove dumplings from steamer; keep warm. Repeat procedure with remaining dumplings.
To prepare sauce, combine 1 tablespoon onions and remaining ingredients. Serve sauce with dumplings.