In a large zip-top bag, toss the chicken with the cornstarch salt and pepper. Shake to coat well.
Heat oil in a large skillet over medium-high heat. Cook chicken in batches for 5-6 minutes or until lightly browned. Transfer chicken to a lightly greased slow cooker.
In a small bowl, whisk together chicken broth, soy sauce, brown sugar, orange juice and zest, vinegar, garlic, ginger, red pepper flakes and sesame oil. Pour sauce over chicken.
Cover and cook on low for 3-4 hours.
About 30 minutes before serving, whisk together the 2 tablespoons of water and cornstarch in a small bowl and stir mixture into the slow cooker. Cover and cook on high for another 20-30 minutes or until the sauce has thickened. Taste and adjust seasonings if needed.
Sprinkle with sesame seeds, green onions and serve over cooked rice.