Start your rice cooking, however much you'd like. (I often do extra so we can have fried rice later in the week.)
Mix the sake and cornstarch together.
Cut the chicken into 1" pieces and then add to to the sake and cornstarch mixture to marinate a bit, at least 10 minutes at room temperature and up to 4 hours she says in the refrigerator. I did about 20 minutes at room temperature.
Heat the oil in a wok or non-stick large pan. Place the chicken in the pan, skin-side down if you have any skin. Let it cook a minute or two until the edges of the meat just begin to turn white. Use tongs (or chopsticks she says) to turn the pieces over and let them brown another minute or two on the other side.
Push all the chicken to one side and add the onion pieces, lowering the heat a bit. Cook about 3 minutes then stir the onion and chicken together. Sprinkle the curry powder and salt over the onion and chicken. Add the stock and stir to break up any crusty bits on the bottom of the pan or wok.
Set the heat so the broth simmers and cook another 2-3 minutes. Add more stock or water if it seems to be too dry. The onion pieces will fall apart and become part of the sauce.
Add the soy sauce and simmer until thickened, about 5 minutes. Taste and add the mirin if you need to. (I did not.)