Step 1Chop the green onions (both white and green parts) and mix them with the ground pork. Grate in a generous bit of ginger. If you found dried shiitake mushrooms, reconstitute a couple, chop them, and add them too; they add a very appealing flavor note. A couple of chopped water chestnuts are also a lovely addition, giving terrific crunch — but only if you can find fresh ones; the canned kind have a nasty metallic taste and a slightly mealy texture.
Step 2In another bowl mix the soy sauce with the rice wine and the sesame oil. Add the sugar, a good grind of black pepper, and the white of an egg. Stir this gently into the pork mixture until it's completely incorporated, and allow it to rest for at least half an hour (or overnight in the refrigerator).
Step 3When you're ready to assemble your dumplings, mix the cornstarch into a half cup of water in a small bowl. Set it next to a pile of dumpling wrappers. I find the commercial wrappers rather thick, so I like to roll each one out a bit with a rolling pin to make it thinner (this also allows you to make fatter dumplings).
Step 4Put a heaping teaspoon of filling onto the wrapper, brush the top edge lightly with the cornstarch mixture, fold the wrapper over into a crescent, and press and print the edges firmly together, trying to press all the air out of each dumpling. Set each one on a baking sheet as it's finished, making sure it's not touching another dumpling. Cover with plastic wrap as you work.
Step 5Freeze the dumplings, in a single layer, on their baking sheet. When they're frozen, put them into plastic bags (they'll keep in the freezer for 6 weeks).
Step 6To cook, bring a big pot of water to a boil. Throw as many dumplings as you'd like into the pot, bring the water back to the boil, and cook for 7 minutes. (If you're cooking unfrozen dumplings, it will take about 5 minutes). They'll rise to the top when they're ready.