Trim the ends of the green beans and cut them diagonally into thirds. Slice the king oyster mushrooms in half lengthwise and then into thin diagonal slices.

Place the beef in a plastic bag with sake, soy sauce, and grated ginger. Massage the mixture into the beef, then coat with cornstarch.

Heat sesame oil in a frying pan over medium heat. Add the beef and stir-fry until the color changes.

Add the green beans and mushrooms to the pan and stir-fry until softened.