Gather all ingredients.

Finely chop the onion and bamboo shoots.

Chop the scallops into small pieces.

In a bowl, combine the chopped scallops, minced pork, chopped onion, and chopped bamboo shoots.

Season with salt, pepper, oyster sauce, sake, and sugar.

Knead the mixture well until it becomes sticky.

Add the corn starch to the mixture and mix well.

Form a circle with your thumb and index finger and place a shumai pastry skin on top of your fingers.

Divide the filling into 20 portions.

Place one portion of the filling onto the center of a pastry skin.

Wrap up the filling by gently pressing the filling into the center of the pastry skin, creating a small indentation on top.

Decorate the top of each shumai with chopped green beans or green peas.

Line a steamer with kitchen paper or parchment paper.

Arrange the shumai in the steamer, leaving generous gaps between each one.

Steam over medium-low heat for 15 minutes, or until the shumai are cooked through.

Serve the shumai hot with soy sauce and Japanese mustard for dipping.