Combine the seasoning mix (kosher salt, paprika, garlic powder, black pepper, and crushed rosemary) in a small bowl.
Brush the turkey breast with melted butter and sprinkle the seasoning mix over top.
Add the celery, carrots and onion to the bottom of a 4-quart slow cooker and pour the broth overtop.
Place the turkey breast in the slow cooker, skin side up.
Cover and cook on low for the cooking times indicated below or until internal temperature reaches 165°F. Do not overcook.
Notes
The skin and be broiled if you'd like a crisp skin.
Cooking times are an estimate and will vary based on the size of the turkey breast. 2 pounds - 2 ¾ to 3 ¼ hours 3 pounds - 3 ¾ to 4 ¼ hours 4 pounds - 4 ¾ to 5 ¼ hours