Remove giblets from turkey, and rinse the turkey well with water. Dry thoroughly.
Cover the turkey breast with the barbecue rub, inside and out, pressing it to stick to the meat and pushing some up under the skin.
Place the onion chunks inside the turkey cavity, and place the turkey skin side up in your slow cooker.
Cover and cook on low 5-7 hours, or until it reaches 165℉ internal temperature.
Let rest, and slice to serve.
Note: To make rub, combine all of the ingredients and mix thoroughly in a large bowl. Store in a dark, cool place in a sealed container.