In a small bowl, thoroughly combine everything except the chicken and chopped onion. Remove giblets from chicken, clean the cavity well and pat dry with paper towels.
Rub the spice mixture into the chicken, both inside and out, making sure it is evenly distributed and down deep into the skin.
Place in a sealed marinating container and refrigerate overnight.
When ready to roast chicken, stuff cavity with onions, and place in slow cooker.
Cook on low for 5 hours or until cooked through. The pan juices will start to caramelize on the bottom of pan and the chicken will turn golden brown when it is ready.
Use a meat thermometer to ensure that the meat is thoroughly cooked (165°F internal temperature).
Let chicken rest about 10 minutes before carving.
Notes
Be sure to allow 10-24 hours for the chicken to marinate.