Place chicken in a single layer on bottom of slow cooker (Crock Pot®)
In a medium nonreactive bowl, whisk together the onion, garlic, soy sauce, rice wine vinegar, pineapple juice, honey, ginger, and white pepper until well combined
Pour teriyaki mixture over the chicken breasts - Cover and allow to cook on high for 4-5 hours (low for 8-9 hours) or until the chicken is cooked through and easily shreds using a fork
Remove the chicken from the slow cooker using a slotted spoon and shred using a couple of forks (how fine you shred it is up to you - I like it kind of 'chunky') and set aside
Transfer the remaining teriyaki liquid from the slow cooker to a medium sauce pan
In a separate bowl, whisk together the water and cornstarch into a smooth slurry
Whisk cornstarch slurry into the teriyaki liquid until well combined
Place the teriyaki/cornstarch mixture over medium-high heat and bring to a boil - Allow to boil for 1-3 minutes or until thickened - Adjust flavor to your liking
Remove from heat and use as desired - You can toss the shredded chicken breasts with the sauce, pour the sauce over the top, pour some over the top and reserve the rest as a dipping sauce... Really up to you!
Serve the shredded chicken (sauced as you like) hot over rice with optional garnish OR use to make teriyaki sliders with a little shredded red cabbage for 'crunch' (I like using King's Hawaiian Sweet Bread Rolls® for the slider buns)