pinchof black pepperand/or gochugaru (red chili pepper flakes)
For the wrappers:
Add 2 cups of flour to a bowl. Dissolve 1/2 teaspoon of salt in 2/3 cup of water. Gradually add the water to the flour while mixing and checking the moisture level. Keep mixing by hand until the dough comes together, and knead with the heel of your hand until the dough is smooth bur slightly stiff. You can adjust the dough by kneading in a little more flour or more water.
Cover with a plastic wrap, and let it rest for 30 minutes to an hour. After resting, the dough should feel soft and smooth. (This is a good time to start making the filling.)
When ready to use, using a sharp knife, cut the dough into 4 long pieces on a lightly floured work surface. Roll each piece with both hands to make a thin log, 3/4 to 1-inch diameter. (Cover the remaining dough to keep it from drying out.)
Cut each log into 3/4 to 1-inch pieces. Press the cut side with the thumbs to flatten to a small disk. Dust and roll out each disk with a small rolling pin to a thin 3.5-inch circle. (You can also use a round cutter or wine glass.) Make a few wrappers at a time and wrap the filling in. Always cover the dough and wrappers that are not being used.
If making green dough
Blanch the spinach in boiling water, puree in a food processor. Run it through a sieve to extract green juice, about 2/3 cup. Follow the same steps for the wrappers above.
For the filling:
Coarsely chop the shrimp or pulse several times in a food processor. Finely chop the cabbage. Mix with 1 teaspoon of salt. Let sit for 5 - 10 minutes. Squeeze out excess water. (If using a food processor, add salt with the cabbage before pulsing.) Finely chop the onion and mushrooms. Combine all the filling ingredients in a large bowl. Mix well by hand.
Place a tablespoon of the filling on a wrapper. You don't need to wet the edges of the homemade wrapper. Seal tightly (pushing the air out with your fingers) into a half-moon shape. (You can add pleats if desired.) Dust the bottom of the dumpling with flour to keep it from sticking as the skin absorbs the moisture from the filling. Repeat this process until all the filling/wrappers are used.
Steaming: steam the dumplings for about 10 minutes in a steamer (longer if frozen). Make sure to line the steamer with a wet cheesecloth or paper towel to prevent the mandu from sticking.
Boiling: bring a pot of water to a boil. Add mandu (stirring gently so they don't stick to the bottom of the pot), a few at a time, and cook until all of them come up to the surface. Continue to cook for another minute or two.
Pan frying, heat the pan with 1 tablespoon of vegetable oil over medium high heat. Add the dumplings, making sure they aren't touching each other. Fry for 1 - 2 minutes, until the bottoms are golden brown. Add 1/4 cup of water to the pan, and cover immediately with a lid. Reduce the heat to medium low, and steam for 4 to 5 minutes. If the dumplings are frozen, cook a little longer.
Tips for freezing: Freeze the dumplings on a tray with no pieces touching for about an hour, and then store them in a freezer bag. Otherwise, the skins will get soggy from the moisture in the filling and stick together in the freezing process.