Chop brown onion finely. Re-hydrate dried Shiitake mushrooms in water, squeeze water out, and chop finely.
In a large bowl, add pork, onion, Shiitake mushrooms, cornstarch and all the seasonings. Mix very well until combined.
Wrap the meat mixture with the Shumai skins (watch video) and shape into little cylinders. Top with a green pea on each dumpling.
Lay lettuce flat in the inside of a steamer. Place Shumai on the lettuce, and cook in a steamer for 10 minutes at medium heat.
Mix soy sauce and vinegar. Serve with sauce and mustard.