Place a 10-inch cast iron skillet on the middle rack of the oven. Heat oven to 450°F (230°C) so the skillet heats along with it. Carefully remove with oven mitts, add olive oil, and swirl to coat the bottom.
Combine Italian seasoning, sage, garlic powder, salt, and black pepper. Rub mixture evenly over both sides of the turkey tenderloin.
Place the tenderloin in the hot skillet. Using oven mitts, return skillet to oven and bake for 10 minutes.
Remove skillet, flip the turkey with tongs, reduce oven temperature to 400°F (200°C), and roast for another 15 minutes.
Remove from oven, top with butter, and cover skillet with foil or a pan lid. Let rest 15 minutes to lock in juices.
Turkey is ready when an instant-read thermometer inserted in the thickest part reaches 165°F (74°C).Remove cover, slice turkey, and spoon melted butter sauce over the top before serving.
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Notes
Season evenly: Coat the tenderloin on all sides for the best flavor.
Add butter for flavor: Melting butter over the hot turkey creates a rich sauce when combined with the pan juices. Spoon it over sliced turkey before serving.
Rest before slicing: Cover the tenderloin and let it rest for 15 minutes. This keeps the meat juicy as the juices redistribute.
Check doneness: Always use a meat thermometer. The turkey is fully cooked when the thickest part reaches 165°F (74°C).
Adjust for size: The recipe works with smaller or larger tenderloins. Just keep an eye on cooking time and use a thermometer to guide you.