Place chicken in a 6- to 7-quart slow cooker. In a bowl or large measuring cup, mix ketchup, water, chopped onion, brown sugar, apple cider vinegar and Worcestershire sauce. Pour half the BBQ sauce over chicken breasts and thighs. Toss to coat completely. Pour remaining sauce into a small saucepan. Simmer 10 to 15 minutes until thickened. Set aside.