Cut the chicken into 1-inch bite-sized pieces. Place the cornstarch in a shallow dish or zip-top bag, add the chicken, and toss until evenly coated.
Heat olive oil in a large skillet over medium-high heat. Once hot, add the coated chicken in a single layer.
Sear the chicken for 2–3 minutes per side until lightly golden. The chicken does not need to be fully cooked at this stage.
Transfer the browned chicken to a 6-quart slow cooker, spreading it out evenly.
In a medium bowl, whisk together the orange marmalade, soy sauce, vinegar, sesame oil, ground ginger, garlic, red pepper flakes, and sesame seeds until smooth.
Pour the sauce over the chicken in the slow cooker. Stir gently to coat all pieces evenly.
Cover and cook on LOW for about 3 hours, stirring once halfway through to ensure even cooking and prevent sticking.
Once the chicken is tender and fully cooked, garnish with chopped scallions and additional sesame seeds. Serve hot over rice or noodles.
