(optional) Marinate the chicken: This step is completely optional but will result in juicer ad tastier chicken. Simply place the sliced chicken and the marinade ingredients in a bowl. Add a splash of water and mix so that the chicken is thoroughly coated. Let this rest for 15 to 30 minutes.
Combine the sauce ingredients in one bowl, and mix the corn starch slurry in a separate bowl. Set both of these aside until later.
Stir fry the chicken: Add roughly 1 to 2 tablespoons of vegetable oil to either a wok or large walled pan. Place this over high heat and let it heat up until the oil is smoking. Then toss in the chicken and stir fry for 3 to 4 minutes or until the chicken is mostly cooked through and slightly browned. Transfer the chicken to a plate and set it aside.
Stir fry vegetables: Add another tablespoon or two of vegetable oil to your wok/pan and place it back over high heat. Toss in the green beans and stir fry for 2 minutes. Then toss in both the onion and the garlic. Continue to stir fry these ingredients for a couple more minutes or until the green beans and onions begin to soften.
Finish the dish: Add the chicken back to your wok/pan and stir fry it with the vegetables for a few seconds. Pour in the sauce and combine it with the other ingredients. Let everything cook for 30 seconds to 1 minute or until the sauce comes to a simmer. Then pour in the cornstarch slurry and stir so that it is incorporated with the sauce. Continue to stir the ingredients until the sauce thickens up and coats everything, about 1 minute.
Serve the green bean chicken immediately with a side of rice and enjoy!