Combine the sauce: In the crockpot, whisk together the soy sauce, brown sugar, rice vinegar, sesame oil, garlic, and ginger.
Add the chicken: Add the chicken thighs and turn to coat in the sauce.
Cook: Cover and cook on low for 5-6 hours or high for 3 hours, until the chicken reaches 165°F and shreds or slices easily.
Thicken the sauce: Remove the chicken to a plate. Whisk the cornstarch and water together, stir into the sauce, and cook on high, uncovered, for 5-10 minutes until glossy and thickened.
Finish: Slice or shred the chicken, return it to the sauce, and toss to coat. Serve topped with sesame seeds and green onions.
Notes
Rice vinegar and sesame oil are what give this real teriyaki depth instead of generic sweet soy flavor. Don't skip the cornstarch slurry.