Cook onions in butter/olive oil over MEDIUM heat until translucent.
Add the remainder of the ingredients, stirring to blend.
Bring to a low boil and reduce heat to simmer, stirring frequently, for about 15 minutes or until sauce has thickened. When you scrape the bottom, there should see a streak (the bottom of the pan) which SLOWLY fills. Remove from heat.
Pour sauce over shredded chicken in crock pot; combine to coat evenly. Keep on warm until ready to serve.