Prepare the rub and set aside.
Remove the pork tenderloin from the package and pat dry. Trim off any excess fat or silver skin, then coat the entire surface of the tenderloin with the rub, packing it in gently, until no meat is exposed.
Pre-heat the oven to 350°F and a cast iron skillet to medium-high heat. When the pan is hot, add the olive oil and one tablespoon of the butter, then immediate add the tenderloin.
Using cooking tongs, rotate the meat to sear three sides, about 1-2 minutes per side. Turn the loin onto the fourth side and immediately turn off the burner. Transfer the skillet to the oven. Roast the loin for 15 minutes or until the internal temperature reaches 145°F.
Transfer the loin to a serving dish or cutting board and allow it to rest for 5 minutes. Meanwhile, melt the remaining butter in the skillet with the pan drippings over medium-low heat. Stir the butter to deglaze the pan and to create a small amount of pan sauce.
Cut the loin into 1-inch rounds and drizzle the pan sauce over the meat before serving.