Place the boneless, skinless chicken thighs (or breasts) at the bottom of the slow cooker.
In a small bowl, whisk together the soy sauce, honey (or brown sugar), rice vinegar, minced garlic, grated ginger, and sesame oil. Pour the sauce mixture over the chicken.
Cover and cook on low for 6-8 hours or high for 3-4 hours until the chicken becomes tender and easily shredded.
If you prefer a thicker sauce, mix the cornstarch and water in a small bowl until smooth. About 30 minutes before serving, stir the cornstarch mixture into the slow cooker and cook uncovered on high.
Once the chicken is fully cooked, use two forks to shred it directly in the slow cooker. Stir it into the sauce, ensuring every piece is coated.
Serve the teriyaki chicken over steamed rice or noodles, garnished with sesame seeds and green onions for extra flavor.