In a medium mixing bowl, combine chicken and marinade ingredients. Let marinate while you prepare the remaining ingredients.
In a small bowl, combine sauce ingredients and set aside.
In a wok or large frying pan on medium high heat, add 2 tablespoons cooking oil, add the chicken and let it sear on one side before flipping over. Sear for another 2 minutes then stir fry until chicken is cooked through, 3-5 minutes. Transfer the chicken into a clean bowl and set aside.
Add remaining tablespoon cooking oil and onion to the wok and stir fry for 2-3 minutes. Add remaining ginger and garlic, stir fry for 30 seconds, until fragrant. Add sauce and let cook for 1 minute.
Meanwhile, in a small bowl, dissolve cornstarch in cool water to make a slurry.
Stir the slurry into the sauce. Add the cooked chicken along with the drippings and cook until the sauce starts to simmer and thicken, and chicken is heated through, about another 1-2 minutes. Turn off the heat. Add green onion and sesame oil.