Preheat oven to 425 degrees.
Season all sides of the tenderloin with Italian seasoning, salt, and pepper.
Heat olive oil in a large heavy-bottomed skillet, such as cast iron, over high heat.
Add the pork to the skillet and sear for 2-3 minutes per side.
Transfer skillet to the oven and bake for 15 minutes or until the pork reaches an internal temperature of 145 degrees.
Remove from oven and let rest for 5 minutes.
While pork is resting, whisk together the butter, garlic, onion powder, and parsely.
Pour the garlic butter over the pork.
Slice pork into 1-inch thick slices and serve.