How to Make It
On a work surface, combine the chopped watercress, cabbage, scallions, garlic and pine nuts and finely chop them all together. Transfer the mixture to a bowl and season with the salt and white pepper. Knead the mixture slightly to wilt the greens.
Spread 6 gyoza wrappers on a work surface and brush the edges with water. Spoon 2 teaspoons of the filling into the center of each wrapper and fold in half; press the edges firmly to seal, pressing out any air pockets. Transfer the dumplings to a cornstarch-dusted baking sheet. Repeat with the remaining wrappers and filling.
In a pot of boiling salted water, cook the dumplings until tender and translucent, about 3 minutes. Alternatively, steam the dumplings on a rack set on a pan over boiling water. Serve the dumplings hot with soy sauce.