Place the chicken thighs in the slow cooker.

In a medium bowl, whisk together the soy sauce, brown sugar, 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 tender and cooked through.

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 this mixture into the sauce in the slow cooker.

Turn the slow cooker to high and let the sauce thicken for about 10-15 minutes.

Return the shredded chicken to the slow cooker and stir to combine with the sauce.

Serve the teriyaki chicken over cooked rice, garnished with sliced green onions and sesame seeds.