Whisk together all ingredients for the sauce, set aside.
Toss the chicken with the cornstarch, salt and pepper, until chicken is fully coated. Set aside.
Heat a sauté pan over medium high heat. Add one tablespoon of oil. Cook the green beans, stirring often, until starting to blister, 3-4 minutes. Remove from pan and reserve on a plate.
Add the second tablespoon of oil to the pan. Add the chicken, but do not overcrowd the pan. Cook in two batches if necessary. Cook chicken 3-4 minutes per side, until cooked through.
Add the green beans back to the pan with the chicken. Pour in the sauce. Stir, scraping up any browned bits on the bottom of the pan. Cook for 2-3 minutes, until chicken and green beans are fully coated and sauce has thickened. Garnish with sesame seeds and serve over rice if desired.