In a small bowl, combine the garlic powder, paprika, salt, pepper, rosemary leaves, and olive oil. Stir well to make a paste.
Pat the turkey breast dry with paper towels, then coat all sides with the paste. Let it marinate for at least 10 minutes, up to several hours, or overnight (covered in the fridge).
Add the chicken broth to the slow cooker and place the onion halves inside, cut-side down. Place the turkey on top of the onions.
Cover and cook the turkey breast on high for 2-3 hours or on low for 4-6 hours, until the meat is cooked through to an internal temperature of 165°F.
Transfer the turkey breast from the slow cooker to a roasting pan or baking sheet, skin-side up. Baste with juices from the slow cooker, if desired, then place it under the broiler for 3-5 minutes to crisp the skin.
Once the skin is crispy, remove from the oven, tent with foil, and let the turkey rest for 10 minutes. Slice and serve warm.