Trim the root of the shimeji mushrooms and separate them into bite-sized pieces. Thinly slice the onion.

In a bowl, combine the pork with sake, soy sauce, and potato starch. Massage the mixture into the meat.

In a separate bowl, mix sugar, vinegar, soy sauce, grated garlic, grated ginger, and potato starch to prepare the sweet and sour sauce.

Heat salad oil in a frying pan over medium heat. Add the seasoned pork and stir-fry until the color changes. Add the shimeji mushrooms and onion, and stir-fry until softened.