Whisk all the marinade ingredients together until well mixed. Pour it on top of the breast, and allow it to marinate for at least 30 minutes or up to overnight in the fridge. When ready to use, drain the pieces of meat well with a slotted spoon and discard the marinade.
Add all the sauce ingredients to a bowl, except for the cornstarch, and whisk to combine. You should have ½ cup of sauce.
Heat up 2 tablespoons of oil in the wok over medium-high heat. Add the meat, but don't crowd the wok, and cook for 3 minutes, stirring constantly. If necessary, cook in batches. Place the meat in a bowl and wipe the wok clean with paper towel.
Heat up the other 2 tablespoons of oil over medium heat and stir fry the string beans and onion for 2 minutes, stirring constantly. Add the cooked vegetables to the bowl of cooked meat.
Whisk the cornstarch with water to make a slurry, then whisk the slurry into the sauce. Pour it into the wok and cook to thicken it up, about 30 seconds to 1 minute. Make sure to stir the sauce constantly to prevent burning.
Add the meat and vegetables back to the wok, stir to coat each piece with the stir fry sauce, and heat it through, roughly 2 to 3 minutes.
Serve hot with white or brown rice and other Panda Express side dishes like chow mein, super greens, fried rice, or eggplant tofu.