Place chicken in slow cooker.
In a mixing bowl, whisk together brown sugar, soy sauce, and rice vinegar. Add ginger, garlic, onion, and black pepper. Whisk well.
Pour the mixture over chicken. Cover slow cooker. Cook on LOW for 6 hours or on HIGH for 4 hours.
When done, DO NOT turn off the slow cooker. Shred chicken with two forks.
In a small measuring cup, whisk cornstarch with cold water. Add to slow cooker and stir. Let cook for 10 minutes and stir again. The mixture should thicken.
Serve over rice with a side of steamed vegetables.