Finely chop the brown onion. Grate the ginger root. In a large bowl, combine the pork, chopped onion, grated ginger, and all the seasonings. Mix well until thoroughly combined. Wrap the meat mixture with shumai skins (see video) and shape into small cylinders.
Heat 1/2 tablespoon of oil in a frying pan over medium heat. Place the dumplings in the pan and cook for a couple of minutes. Add water, cover with a lid, and steam for 3–4 minutes, until the water has evaporated. When the bottoms are nicely browned, drizzle with 1/2 tablespoon of sesame oil. Cook for another minute, then flip the dumplings and cook for 1 more minute.
Serve with soy sauce and Japanese mustard.
