Crockpot Orange Chicken

3 pounds boneless, skinless chicken breasts
1 small onion, peeled and diced
1/2 cup orange juice
3 Tbsp. orange marmalade
1 Tbsp. brown sugar
1 Tbsp. apple cider vinegar
1 Tbsp. GF worcestershire sauce
1 tsp. Dijon mustard
1 Tbsp. cornstarch mixed with 2 Tbsp. hot water
2 Tbsp. grated orange zest

-Grease the crockpot with nonstick cooking spray.  Cut the chicken into bite-sized pieces.  Add the chicken and the onion to the slow cooker.  In a small bowl, mix together the orange juice, marmalade, brown sugar, vinegar, worcestershire sauce, and mustard.  Pour over the chicken in the slow cooker.  Cover and cook on low for 5-6 hours, or until chicken is cooked through.  About 10 minutes before serving, whisk in the cornstarch slurry.  Uncover the slow cooker, turn the temperature to high and continue to cook for 10 minutes to thicken the sauce.  Serve with orange zest sprinkled on top.