Prepare all the precooked filling ingredients: separate and chop the leafy & crunchy parts of napa cabbage. Rehydrate 4 dried shiitake mushrooms in water for about half an hour, squeeze out moisture, and chop into small pieces. Crush the garlic and ginger, chop the onion, and chop the Asian chives (Asian chives will not be precooked).
On medium low heat, add a little oil to a pan and sauté the onions until fragrant. Add the garlic & ginger and mix for one minute. Next add the crunchy parts of the napa cabbage and the shiitake mushrooms. Season with a pinch of salt and sauté for another 3 to 4 minutes.
Add the leafy parts of the napa cabbage and sauté for another minute or two.
Once everything has softened, turn off the heat and let it cool down completely
Mix together the ground pork, shrimp, soy sauce, potato starch, dashida, and sugar until well combined. In order to get a nice bouncy texture, make sure you mix for at least 2 minutes.
Add the precooked filings and the Asian chives. Mix well.
Wet the edges of the dumpling wrappers with some water and put about 1 tbsp of filling in the center (the amount depends on the size of your wrappers). If you like some extra crunch (or chew) on your mandu, add less filling so that the pleats on the wrapper are a little bigger.
Seal the edges.
Shape the dumplings however you'd like. I tend to use the same shape no matter what kind of dumplings I am making (fried, steamed, in soup, etc). See my video on how I fold my dumplings to get this pattern.
If you are not eating all the dumplings right away, you can line them up on some parchment paper and put them in the freezer. Once frozen solid, you can put them into a ziplock bag and save for later (cooking instructions are the same- just let the dumplings thaw on the counter about 15 minutes before cooking).
On medium heat, add a generous amount of oil (2-3 tbsp) to a pan. Fry the dumplings for about 2 minutes on each side.
The dumplings are ready when they are a nice golden brown color on all sides. If there are any raw parts of skin where the pleats are, I usually tilt the pan so that all the oil wells up on one side, then I place the uncooked parts into the oil until nicely fried.
Alternatively, you can steam the dumplings for 7-8 minutes.
Enjoy with dipping sauce (just mix all ingredients together).