Chop the onion finely and coat with katakuriko. Cut 5 mm triangles from the corners of siumai skin (this is optional, but it is easier to wrap the filling).
Combine the pork mince and the ◆ seasonings. Add the onion and mix well. Wrap the filling with the siumai skin. Place and push down the green peas lightly on top.
Grease the bottom of the frying pan with the vegetable oil and arrange the siumai. Pour in the water and after bringing to the boil cover immediately. Steam for about 10 minutes.
When all the moisture is gone from the frying pan and the bottoms of the siumai is crispy, it's done.