Mix the sauce: In a bowl, whisk soy sauce, honey, rice vinegar, mirin, garlic, ginger, chicken broth, sesame oil, and red pepper flakes if using.
Add chicken to the slow cooker: Place the chicken thighs in a single layer if possible. Pour the sauce over the top, making sure every piece is coated.
Cook low and slow: Cover and cook on Low for 4 to 5 hours or on High for 2.5 to 3 hours, until the chicken is fork-tender.
Thicken the sauce: Transfer the chicken to a plate. In a small bowl, whisk cornstarch with cold water to make a slurry.
Pour the cooking liquid into a saucepan (or leave it in the slow cooker if it has a sauté function). Stir in the slurry and simmer for 2 to 4 minutes, until the sauce is glossy and thick enough to coat a spoon.
Shred or leave whole: Shred the chicken with two forks or slice it. Return the chicken to the sauce and toss to coat.
Serve and garnish: Spoon the teriyaki chicken over warm rice.
Top with sliced green onions and sesame seeds. Add steamed veggies on the side for a full meal.