Place a small cast iron skillet in the oven on a middle rack and heat the oven to 450 degrees F (230 degrees C). The skillet will heat along with the oven.
Using oven mitts, remove the skillet from the oven and add olive oil to the pan. Give the pan a swirl so that the oil coats the entire bottom.
Mix together the Italian seasoning, salt, pepper, and garlic powder in a small bowl. Sprinkle the seasoning mix over the pork and rub the spices in with your fingers.
Place the pork in the skillet and using oven mitts, return the skillet to the oven and bake for 10 minutes.
After 10 minutes, remove the skillet from the oven and using tongs, flip the pork tenderloin over. Reduce the oven temperature to 400 degrees F (200 degrees C) and continue roasting another 15 minutes.
Remove the skillet from the oven. Add the butter to the pan and cover with a large plate, a pan lid, or foil. Let the pork rest 15 minutes before serving. The pork is done when its internal temperature registers 145 degrees in the thickest part of the meat.
Remove the foil. Spoon the butter sauce over the pork before slicing.
Notes
Expert Tips
Season the pork on all sides: Make sure the pork tenderloin is completely coated with the spices and feel free to experiment with your favorite spices and spice blends. You can even season the pork simply with just salt and pepper.
Trim fat if necessary: Most pork tenderloins are sold already trimmed of fat. If the one you purchase still has a bit of fat left on, remove it with a sharp knife.
Don't forget the butter: The heat from the pan will melt the butter which will mix with the juices of the pork. This makes for a wonderful sauce to cover the pork.
Be sure to cover the pork before slicing: Letting the pork "rest" for about 15 minutes after cooking is important. It allows the juices to reabsorb back into the meat so you are sure to have the juiciest pork possible.