Microwave Napa cabbage leaves until soft, about 4-5 mins. Slice off the thick white parts if they are too thick. Cool completely. Cut green onions very thinly. Slice ginger root thinly. Set all aside.
Chop the sliced-off cabbage and brown onion finely. Combine the cabbage, brown onion, pork, Panko, egg, and salt in a bowl. Mix very well, then divide and shape the mixture into 8 oblong balls.
Wrap each meatball with cabbage, tucking the end of the leaf under. Place the cabbage-wrapped meatballs in a pot that can accommodate all 8 pieces.
Pour Dashi into the pot, heat it at medium heat, and cook for 10 minutes covered with parchment paper. Add soy sauce, Sake, Mirin, and sliced ginger. Cook for another 10 minutes, basting the stuffed cabbage with the soup from time to time.
Salt to taste, and serve with sliced green onions on top.
