Wash the rice well and then soak in warm water for 30 minutes. Drain and place in a pot with 1 1/4 cups water. Bring to a boil and boil for 1 minute, then cover and simmer for 20 minutes. Turn off and let stand, covered, for 10 minutes or until you need it.
Meanwhile, heat a tablespoon of peanut or vegetable oil in a wok or large skillet over high heat. Add the pork, mushrooms, and shrimp and cook, stirring and chopping to break up the pork. When the pork is no longer pink, add the soy sauce, wine, sugar, and salt and cook briefly. Add the scallions and rice and toss together, mixing until well-combined. Put in the mixture into a large bowl.
In the wok, which should be still hot, add the spinach and stir-fry until wilted. Add to the large bowl and mix well, using your fingers.
To shape the dumplings, fill a small bowl with water and lightly oil a baking sheet (or line with parchment paper). Keeping your hands moist, put a tablespoon of the filling in the middle of the wonton wrapper and gently press the wrapper around the filling, gathering it at the top. I usually put the wonton wrapper in my palm and then curl my hand around it; with my other hand, which is wet, too, I make a little ring and briefly squeeze the top folds of the wrapper together, like you're strangling its neck. This sounds complicated, but it isn't; you'll find your own way. (The dumpling will be open at the top, of course, and there will be little peaks of wonton wrapper that stick up in the air.)
If eating immediately, steam on a wooden steamer, lightly oiled, for about six minutes. If freezing, put the baking sheet in your freezer until the dumplings freeze, and once they're frozen, you can put them in a bag without them sticking together. To defrost, put them directly on the steamer -- if the middle yields when poked with a chopstick, they're ready.