Place the chicken breasts (or thighs) in the slow cooker.
In a medium bowl, whisk together the soy sauce, ketchup, honey, sesame oil, garlic, black pepper (and red pepper flakes, if using). Pour the sauce over the chicken in the slow cooker.
Cover and cook on low for 3-4 hours or high for 1½-2½ hours, or until chicken is cooked through to 165° F. If using boneless chicken thighs, cook on low for 4-6 hours or high for 2-4 hours.
Remove the chicken to a clean bowl or cutting board and let it rest for a few minutes.
Meanwhile, stir together the 1 tablespoon of cornstarch and 1 tablespoon of water in a small bowl. Pour the cornstarch mixture into the sauce in the slow cooker and stir. Cook on high for 10-20 minutes, until sauce thickens slightly.
Shred the chicken and return it to the sauce in the slow cooker.
Serve chicken and sauce over rice, garnished with sesame seeds and chopped green onions.