Place the chicken thighs in the slow cooker.
In a separate bowl, whisk together the soy sauce, brown sugar, honey, rice vinegar, sesame oil, garlic, ginger, and red pepper flakes (if using).
Pour the sauce over the chicken in the slow cooker.
Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the chicken is cooked through and tender.
Once cooked, remove the chicken from the slow cooker and shred it using two forks.
In a small bowl, mix the cornstarch and water to create a slurry.
Stir the slurry into the remaining sauce in the slow cooker.
Cover and cook on high for an additional 15-20 minutes, until the sauce has thickened.
Return the shredded chicken to the slow cooker and stir to coat with the thickened sauce.
Serve the teriyaki chicken over rice or noodles, garnished with sliced green onions and sesame seeds.