Place the chicken pieces in the bottom of a 6-quart slow cooker.
In a bowl, whisk together the orange juice, orange zest, soy sauce, honey, brown sugar, rice vinegar, garlic, ginger, and red pepper flakes until smooth.
Pour the sauce over the chicken and stir to coat evenly.
Cover and cook on LOW for 3–4 hours or HIGH for 2 hours, until the chicken reaches an internal temperature of 165°F.
Whisk the cornstarch and cold water into a slurry, then stir it into the slow cooker.
Cook uncovered on HIGH for 15–20 minutes, until the sauce is thick, glossy, and coats the back of a spoon.
Serve over rice and garnish with sesame seeds and sliced green onions.