In a skillet saute onion in oil over medium-high heat until tender. Add the chicken breasts and brown on all sides. Carefully add cola, ketchup, garlic powder and pepper. Mix gently and cover.
Simmer 25-30 minutes or until the juices run clear.
Remove the chicken and keep warm.
In a small bowl combine cold water and cornstarch. Whisk until smooth. Add to the skillet - bring to a boil cook and stir for 2 minutes or until thickened.
Return the chicken to the pan and heat through.