For the dumpling filling:
1/2 pound napa cabbage (about 1/4 head), plus extra leaves for lining the steamer
1/2 pound ground pork
1/2 bunch scallions (green and white parts), finely chopped
1/2 cup chopped garlic chives
1 large garlic clove, finely chopped
1 (1-inch) knob fresh ginger, grated
3 tablespoons soy sauce
1/2 teaspoon Asian sesame oil
1/2 pound round wheat dumpling wrappers
For the dipping sauce:
1/4 cup soy sauce
2 tablespoons rice vinegar
1 teaspoon thinly sliced scallions, white part only
1/4 teaspoon toasted sesame seeds
1/2 teaspoon freshly ground black pepper
To make the filling: Coarsely chop the cabbage, and then transfer it to a food processor. Pulse until the cabbage is finely chopped, but not puréed. Remove the cabbage, wrap it in a clean kitchen towel, and squeeze it to remove any excess liquid. Transfer the cabbage to a large bowl and stir in the pork, scallions, garlic chives, garlic, ginger, soy sauce, and sesame oil. If you wish to taste for seasoning, poach or fry a small amount of the filling, and adjust the seasonings to your liking.
To shape the dumplings: Holding a dumpling wrapper flat in your hand, place 2 teaspoons of the filling in the center of the wrapper. Wet the edges of the wrapper and fold the dumpling in half to form a half-moon shape, pressing out the air as you seal the dumpling. Use your fingers to pinch pleats around the edge of the half circle. Repeat until all the filling is used.
To cook the dumplings: Here are three different ways to prepare dumplings.
Boil: Boiling dumplings is the most traditional way to cook them. To do so, bring a large pot of water to a slow boil. Add the dumplings and cook until they float to the surface. Then add about a cup of cold water. When the water returns to a boil, add another cup of cold water. When the dumplings float to the surface again, use a slotted spoon to remove them to a platter.
Steam: Pour about 2 inches of water into a wide pot or a wok, and bring to a boil. Arrange the dumplings in a single layer in a bamboo steamer lined with the extra napa cabbage leaves or parchment paper. Cover the steamer, place it in the pot (don't let the water touch the dumplings), and cook at a low heat for about 5 minutes, until cooked through.
Pan-fry: Heat a nonstick pan over high heat. Add 2 tablespoons vegetable oil and swirl it around; then add the dumplings in one layer without letting them touch. Add enough water to reach halfway up the dumplings. Cover the pan a