Combine the sauce ingredients in a bowl, stirring until the cornstarch and brown sugar are dissolved, and set aside.
Sprinkle the cut up chicken with the cornstarch and toss to coat.
Heat a wok or heavy frying pan over high heat. Once hot, add the oil.
Add the chicken and fry until cooked through, about one minute. Add the garlic and ginger and fry for another 30 seconds.
Add the sauce and white parts of the green onions and simmer until thickened, about a minute. Add the remaining green onions, stir to coat and remove from heat (the green onions should remain a little crispy).