Measure & prep shumai ingredients according to the ingredients list, e.g. slice green onions, dice mushrooms, chop water chestnuts, measure liquid ingredients, etc. To prepare the cornstarch slurry, simply whisk it together in a small bowl or liquid measuring cup (I typically combine the cornstarch & water in a jar & vigorously shake to combine). Set aside.
Place ground pork in a large bowl, pouring the cornstarch slurry & sesame oil over top. Using your hands, mix to combine very well, until the pork has a sticky appearance. Add in the remaining filling ingredients. Using your hands, mix to combine very well, until everything is evenly combined. Set aside for dumpling assembly, or store in an airtight container in the refrigerator.
Line a quarter sheet pan with wax paper or parchment paper & set aside. To wrap & fold the shumai dumplings, place a rounded tablespoon of the pork mixture in the center of one of the round shumai wrappers. Use the back of the spoon to spread the pork to the edge of the wrapper, leaving a ⅛-inch border along the edge.
Bring the pointer finger & thumb of your non-dominant hand together, making a circle. Lay the prepared shumai wrapper over top, meat side facing up, & press the middle of the wrapper down slightly to raise the edges up. Using your dominant hand, begin pinching the edge of the wrapper & laying it back, working your way around until you form the “purse” shape signature to siumai dumplings. Use the bottom of your spoon to smooth & flatten the exposed pork mixture at top of “purse," then use your thumb to flatten bottom of “purse.” Set on the prepared baking sheet.
Repeat dumpling assembly until you've used all of the remaining pork mixture. Transfer them to the refrigerator while you prepare your steamer pot, or transfer to a parchment-lined airtight container & store in the refrigerator.
Prepare your steamer pot according to manufacturers directions. Very lightly spray the steamer baskets with nonstick cooking spray, then place the siu mai in the prepared baskets. Once the water reaches a boil, place the steamer baskets over top & cover with a lid. Steam for about 7 minutes, until the wrappers are soft & the pork is cooked through. Transfer to a serving plate & repeat with any remaining siumai.