Season the chicken thighs or breasts with salt and pepper on both sides.
In a bowl, whisk together the honey, soy sauce, minced garlic, ketchup, rice vinegar (if using), and red pepper flakes (if using).
Place the chicken in the bottom of the crockpot.
Pour the sauce over the chicken, ensuring each piece is coated.
Cover the crockpot and cook on low for 4-5 hours or high for 2-3 hours, until the chicken is tender and fully cooked.
If you prefer a thicker sauce, remove the chicken from the crockpot and transfer the sauce to a small saucepan.
Bring the sauce to a simmer over medium heat and cook for 5-7 minutes until slightly reduced.
Pour the thickened sauce back over the chicken.
Transfer the chicken to a serving dish and spoon the sauce over the top.
Garnish with sesame seeds and chopped green onions if desired. Serve with your favorite side dishes.