Place chicken in crock-pot. Sprinkle onions over the chicken.
In a separate bowl, combine soy sauce, sugar, vinegar, garlic, ginger, and pepper. Pour sauce over chicken.
Cover crock-pot and cook on low heat for about 4-5 hours or until chicken is tender. Check after 4 hours.
Remove chicken from crockpot.
Skim fat from cooking liquid, if there is any.
To serve: Top 1 piece of chicken with a little sauce. Serve with 1 small baked potato or ½ cup brown rice. Mash some teriyaki sauce into the baked potato and drizzle with little more teriyaki sauce over the top. If using rice, place chicken on top of rice and drizzle teriyaki sauce all over the top of chicken and rice. In addition, the chicken could be shredded and topped on the potato or rice.