Lay the onion on the cutting board, flat-side down. With your sharp knife, slice 2-3 horizontal slits in the onion towards the root end. Then slice vertical slits in ⅛ inches.
Then cut perpendicular to the previous slices you made. To finely mince, run your knife through them in a rocking motion.
Put the finely minced onion in a large bowl and add potato starch/cornstarch. Mix together.
With a knife, scrape off the skin of the ginger and grate it. You will need 1 tsp of grated ginger.
In a medium bowl, combine the ground pork, 1 tsp of grated ginger, sake, and sesame oil.
Add sugar and soy sauce.
And salt and pepper.
Using your hands (or with plastic food gloves), knead the meat mixture until they are sticky, like a pale paste.
Then transfer the meat to the onion.
Mix the onion and meat together until well-combined.
Prepare the wonton wrappers, a baking sheet lined with parchment paper, a 1-Tbsp measuring spoon, and a butter knife. Scoop the filling into the measuring spoon, and level the meat off with the knife. Make sure the wonton wrappers are covered in plastic wraps or damp towel while you stuff the filling in so they won't get dry.
Transfer the filling to the center of a wonton wrapper.
Using your left hand (if you’re a righty), make a round hole by connecting your thumb and index finger with the rest of fingers next to each other. Place a wonton wrapper with the filling on top of it. Tip: Once you get used to making Shumai, you can place the wrapper and then directly put the filling on top, instead of using a measuring spoon.
Using the butter knife or the back of a teaspoon, gently press the filling down while holding the fingers firmly. After you press the meat down, rotate the Shumai 45 degrees so the meat is evenly distributed in the center of the wrapper. Finally, smooth out the surface of the meat.
Your left fingers should look like this while you press the meat inside, the ring finger and pinky right underneath the bottom of the Shumai to create the flat base. Continue with the rest of the wrappers and filling until one of them runs out.
When you finish wrapping, bring 1-2 cups of water in a wok (or steamer) to boil over high heat (or medium-high heat if you’re using non-stick). Make sure the water does not touch the bottom of the bamboo steamer basket. Meanwhile, place a green pea in the center of the Shumai. Tip: If you don’t have a steamer, read my blog post above on how to steam Shumai without a steamer.
Prepare a parchment paper (make a few air vent holes) or cabbage leaves and place them on the bottom of the steamer basket to prevent the Shumai from sticking. Put Shumai on top without touching each other. Most likely, you will need to steam them in batches. When the water is boiling, place the bamboo steamer basket onto the wok and steam for 8-10minutes until the meat is cooked through.
After 8-10 minutes, check to see if the meat is cooked through. Then transfer the bamboo steamer basket on top of a large plate.
To Serve and Store
Bring to the table and serve immediately with soy sauce, rice vinegar, Japanese chili oil (la-yu) and Japanese karashi mustard. You can freeze before or after steaming them. Place each Shumai on a baking sheet lined with parchment paper without touching each other, and flash freeze for 1 hour. Once frozen, transfer to a freezer bag. You can steam frozen Shumai without thawing (just need to steam for an extra few minutes).