Thaw the turkey breasts completely in the refrigerator or cold water.
Prepare the marinade by mixing softened butter, garlic powder, onion powder, thyme, rosemary, salt, and black pepper.
Loosen the skin of each turkey breast and spread half of the marinade under the skin and the rest on the outside.
Preheat the oven to 325°F (165°C) if roasting, or prepare the slow cooker.
Cook the turkey in the slow cooker on low for 6-8 hours or roast in the oven for 2-2.5 hours until the internal temperature reaches 165°F (75°C).
Let the turkey rest for 10-15 minutes before slicing and garnish with fresh parsley.