Prep the chicken: Trim excess fat. If using bone-in thighs with skin, remove the skin to avoid a greasy sauce.
Pat dry with paper towels.
Make the sauce: In a bowl, whisk together honey, soy sauce, minced garlic, vinegar, ketchup, sesame oil, ginger, and red pepper flakes if using.
Add to slow cooker: Place the chicken thighs in the slow cooker in a single layer as much as possible. Pour the sauce evenly over the top, coating all pieces.
Cook low and slow: Cover and cook on Low for 4–5 hours or High for 2.5–3.5 hours, until the chicken is tender and reaches 165°F internally.
Thicken the sauce (optional but recommended): Transfer the cooked chicken to a plate. In a small bowl, stir cornstarch with water or broth to make a slurry.
Pour the slow cooker juices into a saucepan, bring to a simmer, and whisk in the slurry. Cook 1–3 minutes until glossy and slightly thick. Alternatively, turn the slow cooker to High, stir in the slurry, and cook covered 15–20 minutes.
Finish and serve: Return the chicken to the thickened sauce or pour sauce over the chicken.
Garnish with sliced green onions and sesame seeds. Serve over rice, noodles, or steamed broccoli.