Like most dumplings, raw shumai can be made ahead and stored frozen. To do this, sprinkle the sides and bottom of a tray with flour. Arrange the dumplings in the tray with a small gap in between. Freeze for 2 to 3 hours. Transfer to a zip bag or airtight container. This recipe should last for up to 1 month. To cook frozen shumai, steam for 12 minutes over high heat. No need to thaw.