Make the filling: In a bowl, combine well the scallion, the pork, the ginger, the bell pepper, the cabbage, the garlic, the oil, the soy sauce, the coriander, the egg, and salt and pepper, to taste, and chill the filling for at least 1 hour or overnight.
Put about 1 heaping teaspoon of the filling in the center of 1 of the wrappers and moisten the edge of the wrapper. Gather the edge of the wrapper up and around the filling and form a waist with the wrapper, pushing the dumpling from the bottom and keeping the filling level with the top of the wrapper. (The filling should not be enclosed.) Continue to make dumplings with the remaining wrappers and filling in the same manner and arrange them in 1 layer on a baking sheet lined with waxed paper dusted lightly with the cornstarch. (The dumplings may be prepared up to this point 8 hours in advance and kept uncovered and chilled or 1 month in advance and kept covered tightly and frozen. If the dumplings are frozen, do not thaw them in advance.)
In a large non-stick skillet, heat 1 tablespoon of the oil over high heat until it is hot but not smoking and in it fry half the dumplings, flat sides down, over moderately high heat for 1 minute, or until the undersides are golden. Add 1/2 cup water and steam the dumplings, covered, over moderate heat for 3 minutes, or until the pork is cooked through. (If using frozen dumplings, fry them, frozen, for 1 minute, or until the undersides are golden, and steam them, adding 3/4 cup water per batch, covered, for 8 to 10 minutes, or until the pork is cooked through.) Add the remaining 1 tablespoon oil to the skillet and cook the remaining dumplings in the same manner. Garnish the dumplings with the coriander sprigs and serve them with the sauce.
In a bowl whisk together the nam pla, the lime juice, the vinegar, the sugar, the gingerroot, the mint, the coriander, salt and pepper, to taste.
Available at East and Southeast Asian markets, specialty foods shops, and many supermarkets