Whisk together honey, soy sauce, minced garlic, apple cider vinegar, sesame oil, red pepper flakes (if using), and water or chicken broth in a medium bowl.
Pat chicken dry. Sear chicken in olive oil for 2-3 minutes per side until lightly browned, then place it into the slow cooker. If skipping, place uncooked chicken directly into the slow cooker.
Pour the prepared honey garlic sauce evenly over the chicken in the slow cooker.
Cover and cook on LOW for 3-4 hours or on HIGH for 2-3 hours until the chicken is cooked through and easily shredded.
Remove the cooked chicken from the slow cooker and set aside. Whisk together cornstarch and cold water in a small bowl to create a slurry.
Pour the cornstarch slurry into the sauce in the slow cooker, whisking constantly. Cook on HIGH for another 15-20 minutes, or until the sauce has thickened.
Shred the chicken (if desired) and return it to the thickened sauce in the slow cooker. Stir to coat.
Serve hot, garnished with fresh chopped green onions and a sprinkle of sesame seeds, if desired.