Place the chicken into the crockpot and sprinkle salt and pepper on each side.
In a medium bowl, whisk together the ketchup, honey, soy sauce, ketchup, oil, garlic, diced onion, and red pepper flakes. Pour the sauce over the chicken in the crockpot.
Cook on LOW for 3-4 hours or HIGH for 1½ to 2 hours.
Remove the cooked chicken from the crockpot and cut it into bite-size pieces.
In a small bowl, whisk the cornstarch and water then stir it into the crockpot to thicken the sauce. Add the chicken pieces back into the crockpot then stir into the sauce.