Grease or spray the inside of the slow cooker. (I use oil and spread with a pastry brush.)
Cut chicken into bite sized pieces and add to slow cooker.
In a medium sized bowl, combine honey, soy sauce, ketchup, onion, oil, garlic, salt, pepper, and pepper flakes if using. Pour over chicken.
Cook for 2 ½ - 3 ½ hours on low, or 1 ½ - 2 hours on high, until chicken is just cooked. Check after the lowest time and stir chicken well (it may stick together).
In a small bowl, mix together cornstarch and water*. Pour into slow cooker, and quickly stir to combine. Replace lid, set slow cooker on high, and cook sauce for ten minutes, or until slightly thickened**.
Serve and sprinkle with sesame seeds and green onion.
Notes
*If you typically prefer a thinner sauce, mix together 1 ½ Tablespoons cornstarch and 1 ½ Tablespoons water. Pour into slow cooker, and quickly stir to combine. Replace lid, set slow cooker on high, and cook sauce for ten minutes, or until slightly thickened. If you’d like to thicken further, mix remaining cornstarch and water together and mix into the sauce. Cook for 5 minutes
**If the sauce is not as thick as you’d like, mix ½ Tablespoon cornstarch and ½ Tablespoon water together and mix into the sauce. Cook for 5 minutes.