In a small bowl, mix together the black pepper, salt, paprika, garlic powder, onion powder, dried herb blend, and oil to make a paste.
Use some paper towels to pat the turkey breast dry. Then, scoop the spice-and-herb paste onto the turkey and rub it in on all sides, paying special attention to the skin on top.
Arrange the onion and garlic halves cut-side down in a slow cooker, and top them with the seasoned turkey breast, skin-side up. Cover and cook on low for 5 to 6 hours or until the internal temperature of the turkey reaches 165°F on an instant-read thermometer. If possible, check the internal temperature of the turkey after 4½ hours of cooking; mine was slightly over 165°F at the 5-hour mark.
As soon as the turkey is done cooking, remove it from the slow cooker and let it rest on a carving board for about 15 minutes. If you’d like to crisp up the turkey skin, preheat your broiler to high and set a rack in the center of your oven. Otherwise, remove and discard the turkey skin, and cover the turkey with foil to keep it warm.