In a small bowl, mix the garlic powder, onion powder, paprika, kosher salt, and black pepper.
Pat the turkey breast dry with paper towels. Rub the turkey all over with the olive oil, then coat evenly with the spice mixture.
Place the halved garlic head and quartered onion at the bottom of the slow cooker. Pour the chicken broth around the vegetables.
Place the seasoned turkey breast on top of the garlic and onion mixture.
Cover and cook on LOW for 5–6 hours or on HIGH for 3–4 hours, until the internal temperature reaches 165°F.
Optional: Carefully transfer the turkey to a baking sheet and broil for 5–10 minutes to crisp the outside.
Let the turkey rest for 10 minutes before slicing to retain the juices.