In a medium size bowl, combine the ingredients for the marinade and add in the thinly sliced chicken breast. I recommend letting this marinate in the fridge for at least 30 minutes to overnight. Otherwise, let this sit while you prep the other ingredients.
Add all the ingredients for the sauce in a bowl and set aside.
Heat a wok or pan over high heat and add oil. Wait until the pan is hot and then lay the chicken pieces flat in a single layer - leave out the marinade. In order to get a good sear, don't crowd the pan and leave the chicken pieces alone until you get a golden brown color, anywhere from 30 seconds to 1 minute. Flip over the chicken and repeat. Remove the chicken from the pan.
Add the green beans and white onions to the pan and saute for 2 - 3 minutes. Lower the heat to medium and add the chicken back in.
Next add in the sauce - use about half if you prefer less sauce(stir the sauce before adding it as the cornstarch will settle). Saute everything together for 1 - 2 minutes until the sauce has thickened. Top with sesame seeds. Serve immediately.
Store leftovers in an airtight container and keep in the fridge for up to 3 days.