Add chicken, onion, and garlic to the base of the slow cooker.
Whisk 1/2 cup of water, soy sauce, honey, brown sugar, rice vinegar, black pepper, and ginger. Pour over top of chicken, ensuring the sauce gets under and all around the chicken. Slow cook on low for 5-6 hours or until chicken is easily shreddable with a fork.
Remove chicken from the slow cooker and shred with 2 forks. Set aside and turn off the slow cooker.
Pour the sauce into a medium sauce pan over medium high heat. In a small bowl, whisk together the remaining 1/4 cup of water and cornstarch. Slowly whisk the cornstarch slurry into the sauce. Continue to whisk and let it boil until the teriyaki sauce begins to thicken, about 2 minutes.
Return the shredded chicken to the slow cooker, pour the sauce over it, and toss until evenly coated.