In a small bowl, place the butter, garlic powder, onion powder, paprika, dried rosemary or thyme, salt, and pepper. Stir until combined.
Loosen the skin of the turkey breast. Spread half of the butter mixture under the skin and spread the other half over the top of the turkey.
Spray the slow cooker with cooking spray and place the onions, carrots, and celery on the bottom.
Place the turkey breast on top. Cover and cook on low for 6 hours or until the thickest part of the breast reaches 165 degrees F.
Transfer the turkey breast to a baking sheet. Broil the turkey for 3 to 5 minutes or until the skin is browned and crispy to your liking. Keep a close eye on it to prevent it from burning.