Preheat the oven to 350° F
Put the chicken pieces in a heavy, resealable plastic bag.
Mix the mirin, soy sauce, and grated ginger in a small bowl. Pour over the chicken pieces and massage gently to distribute the marinade.
Squeeze out the excess air and seal the bag well. Let sit in the refrigerator overnight (or at least several hours)
Heat the oil in a wok or large, nonstick frying pan over very high heat. When the oil is very hot, add the chicken. Stir-fry quickly until the pieces are lightly browned or have turned opaque on the outside
Put the pieces in a baking dish, and cover loosely with lightly oiled wax paper (which should sit inside the dish and directly on top of the chicken pieces) and bake for 8 - 10 minutes or until the chicken pieces are just cooked through.
(If not to be eaten immediately, remove the pieces from the hot baking dish to prevent them from drying out)
Let rest for 5 minutes before serving