In a large skillet, saute onion in oil until tender. Add chicken; brown on all sides. Carefully add cola, ketchup, garlic powder, salt and pepper. Cover and simmer for 25-30 minutes or until a thermometer inserted in the chicken reads 170°. Remove the chicken and keep warm. , Combine the cornstarch and cold water until smooth; gradually add to the skillet. Bring to a boil; cook and stir for 2 minutes or until thickened. Return chicken to pan; heat through.