Prep the chicken: Trim excess fat from chicken thighs or breasts.
Pat dry with paper towels for better sauce cling.
Mix the sauce: In a bowl, whisk soy sauce, brown sugar, honey, rice vinegar, mirin, garlic, ginger, sesame oil, black pepper, red pepper flakes (if using), and chicken broth.
Load the crockpot: Place the chicken in the slow cooker. Pour the teriyaki mixture over the top, ensuring the chicken is mostly submerged.
Cook low and slow: Cover and cook on Low for 4–5 hours or on High for 2.5–3 hours, until the chicken is cooked through and tender. Thighs are more forgiving than breasts.
Thicken the sauce: Remove chicken to a cutting board and tent with foil.
In a small bowl, whisk cornstarch with cold water. Stir the slurry into the hot cooking liquid, then cover and cook on High for 10–15 minutes, until the sauce turns glossy and thick.
Shred or slice: Shred the chicken with two forks or slice it against the grain. Return it to the crockpot and toss in the sauce.
Finish and serve: Taste and adjust seasoning.
Add a splash of vinegar if you want more brightness or a pinch more sugar for sweetness. Garnish with green onions and sesame seeds. Serve over rice or noodles with steamed broccoli or snap peas.