Chop the napa cabbage. Follow the next step carefully, or your uncooked potstickers will be soggy and will tear as cabbage releases its water content when it cooks.
Put the chopped cabbage into a bowl together with one teaspoon of salt, and toss. Salt draws the moisture out of vegetables. Let it stand for between 20 and 30 minutes, then squeeze it down several times to take out as much water as possible. Put to one side.
If you're using dried Shiitake mushrooms, re-hydrate them using hot water for 15 minutes. After that, squeeze the water off and finely chop them.
Roughly chop the mushrooms (if you're using the fresh ones), garlic, ginger, and green onions and place them into one side of your bowl. On the other side of the bowl, add the egg yolk and give it a little whisk. Now add the shredded cabbage, the ground pork, and the rest of the filling ingredients.
Mix the fillings well until everything is evenly combined. Next, cover the bowl with plastic and either flash freeze for 15 minutes or chill for 30. (You can chill for up to 24 hours if time isn't an issue). Chilling the filling makes the mixture less wet and also makes the stuffing easier to work with. That way, you won’t finish up with soggy potstickers.