Finely chop the onion and leek. Combine with the ground pork and mix well with your hands.
Add the rest of the ingredients except for the shumai skins, and knead together well.
Put 1 tablespoon of the filling in the middle of each shumai skin, and fold in the 4 corners to wrap it in. Tap the bottom against a flat surface to flatten it to finish.
Line the formed shumai dumplings in a steamer. Leave a little space in between them since they will expand a bit.
I use a microwave steaming bowl I got at a 100 yen shop, but if you're using a conventional stove top steamer, it takes about 13 minutes to cook over medium heat.
Serve with Japanese mustard and soy sauce.
Story Behind this Recipe
My mother has been making these shumai for a long time.
When filling the dumplings, I use two disposable chopsticks to neaten up the shape. Even if the dumplings are a bit misshapen, after they are steamed you won't notice it much. If you are steaming them in a microwave, the cooking time varies depending on how many you're cooking. I think it's about 4 to 5 minutes per 10 shumai, covered with plastic wrap.