Take minced onion or long onions and microwave for 1 minute. Let the onions cool, then sprinkle flour on the onions and mix the flour and onions together.
In a medium sized bowl, mix ground pork, onions, ginger, sesame oil, oyster sauce, sake, salt and pepper
To shape shumai, make a circle with your index finger and thumb. Take a sheet of won ton wrapper and place it on top of the circle. Place 2 tablespoons of meat filling between your thumb and your index finger and rotate, gently squeezing the shumai. Flatten the base of shumai and the top with a knife. Fold over any excess skin and top shumai with peas.
Boil water in a pot or pan. Line the bottom of steamer with piece of parchment paper or place Chinese cabbage. Place shumai and vegetables in a bamboo steamer, leaving some space between each shumai. Put cover on the steamer and place steamer on the pot or pan and steam for 15 minutes.