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1. To prepare the vegetables: For onions, halve the onion and parboil it in boiling water for one to two minutes, until the layers of the onion have just turned translucent but are not soggy. Let cool; then finely chop. For leeks, separate into two sections – the white, tenderer section and the upper green, tougher section. For the green sections, parboil for one minute and then finely chop.
2. Use a few leaves of Napa cabbage or bok choy for additional moisture. To prepare, parboil for thirty seconds to one minute, until the vegetables are slightly softened but not yet translucent. Finely chop.
3. Combine all of the ingredients in a large bowl, mixing thoroughly with your hands or a wooden spoon. Let the filling rest for thirty minutes before using to fully allow the meat to develop its flavors. The filling may also be kept in the refrigerator for up to 24 hours, or frozen indefinitely.
4. Place approximately 1 tablespoon of filling into the center of each wrapper. Gather the wrapper together by pinching the two sides together, making pleats on just one side of the wrapper in the same direction, until you have enclosed the filling entirely. Sprinkle a board or plate with flour. Place finished dumplings on the plate or board and cover with a moist tea towel.
5. Steam the dumplings for 7 to 9 minutes over medium-high heat, until the skins are semi-translucent. If boiling, bring a large pot of water to a vigorous boil and cook for 5 to 8 minutes, until the skins have just turned opaque. Serve immediately with vinegar and chili-paste on the side.

source: http://www.seriouseats.com/recipes/2009/09/lamb-dumplings-recipe-asian-cooking.html