In a bowl, whisk together soy sauce, honey, ketchup, hoisin sauce, vinegar, garlic, ginger, sesame oil, and chili flakes.
Place chicken thighs in the slow cooker and pour the sauce over the chicken.
Cover and cook on LOW for 6–7 hours or on HIGH for 3–4 hours, until the chicken is tender.
Remove chicken from the slow cooker and shred it with two forks.
In a small bowl, mix cornstarch and water to make a slurry. Stir it into the sauce in the slow cooker.
Return shredded chicken to the slow cooker and stir to coat. Let cook for another 10–15 minutes on HIGH until the sauce thickens.
Serve over white rice, topped with green onions and sesame seeds.