- In a small bowl, combine softened butter, garlic powder, onion powder, smoked paprika, dried thyme, salt, and black pepper to form a herb butter paste. 
- Pat the turkey breast dry. Carefully loosen the skin and spread half the butter mixture under the skin, and the rest over the top. 
- Place the onion slices at the bottom of the slow cooker, then place the seasoned turkey breast on top. 
- Cover and cook on low for 5 to 6 hours, or until the internal temperature reaches 165°F (74°C). Check temperature around 5 hours to avoid overcooking. 
- Optional: Preheat the broiler. Transfer the turkey to a baking sheet and broil for 3-5 minutes until skin is crispy and golden. 
- Remove turkey and let rest for 10 minutes before slicing. 
- To make gravy: strain cooking juices into a bowl, discard solids. Melt butter in a saucepan, whisk in flour, cook 1 minute. Gradually whisk in drippings and chicken broth, cook until thickened (3-5 minutes). Season with salt and pepper. 
- Slice turkey and serve with homemade gravy.