Add the chicken to a large mixing bowl; toss with cornstarch until evenly coated.
Heat olive oil in a 12 inch skillet over medium-high heat. Add the chicken and sauté 3-4 minutes per side. *For best results, don't overcrowd the pan and be sure to leave the chicken undisturbed while browning. Transfer the cooked chicken from the pan to a large bowl or plate.
In the same pan, sauté the scallion pieces at medium-high heat for 30 seconds - 1 minute, until they start to brown. Add the chicken back into the pan.
Combine the sauce ingredients in a large measuring cup or bowl; mix well. Pour the sauce over the chicken and scallions in the pan.
Increase heat to high and bring to a boil. Boil 1-2 minutes, until sauce starts to thicken. Remove from heat.
Garnish with sesame seeds, crushed red pepper flakes, and additional scallions (optional) to taste. Serve over rice.