Place onion, garlic, soy sauce, ketchup, vinegar, salt and pepper in the slow cooker and mix well
Place chicken breasts in the slow cooker and toss to coat with the sauce
Place the lid on the slow cooker and set on low
After about 3 hours, turn the chicken pieces over and continue cooking for another 3 hours
After about 6 hours (total), turn off the slow cooker and with 2 forks, shred the chicken and mix with the sauce
Serve on rolls or over rice