Start by making the dough for the dumpling wrappers. Put the flour in a large mixing bowl. Gradually add the water to the flour and knead into a smooth dough. This process should take about 10 minutes. Cover with a damp cloth and let the dough rest for an hour.
In the meantime, make the filling. In a wok or large skillet over medium high heat, add 3 tablespoons oil and add the ginger. Cook for 30 seconds, until fragrant. Add the onions and stir-fry until translucent.
Add the chopped mushrooms and stir-fry for another 3-5 minutes, until the mushrooms are tender and any liquid released by the mushrooms has cooked off.
Add the cabbage and carrots and stir-fry for another 2 minutes, until the veggies are tender and all the liquid released has been cooked off. Transfer the vegetable mixture to a large mixing bowl and allow to cool.
To the bowl, add the chopped chives, white pepper, sesame oil, shaoxing wine, soy sauce, and sugar. Season with salt to taste (though the soy sauce will usually add enough salt to the filling), and stir in the last 1/4 cup of oil.
To assemble the dumplings, cut the dough into small tablespoon-sized pieces. Roll each out into a circle, and pleat the dumplings (see this post for step-by-step photos on how to fold a dumpling). Continue assembling until you've run out of filling and/or dough.
To cook the dumplings, steam them or pan-fry them. To steam, put the dumplings in a steamer lined with a bamboo mat, cabbage leaf, or cheese cloth, and steam for 15-20 minutes.
To pan-fry, heat 2 tablespoons oil in a non-stick pan over medium high heat. Place the dumplings in the pan and allow to fry for 2 minutes. Pour a thin layer of water into the pan, cover, and reduce heat to medium-low. Allow dumplings to steam until the water has evaporated. Remove the cover, increase heat to medium-high and allow to fry for a few more minutes, until the bottoms of the dumplings are golden brown and crisp.