1. Place beef tenderloin on a sheet of aluminum foil and rub all over with 2 tbsp. oil, rosemary, and garlic. Season tenderloin generously with salt and pepper, then let beef sit at room temperature for 1 hour, allowing seasonings to penetrate meat.
2. Heat oven to 425°. Heat remaining oil and butter in a 12" ovenproof skillet over medium-high heat. Carefully place tenderloin in skillet and cook, turning as needed, until beef is browned on all sides, about 7 minutes.
3. Transfer skillet to oven and cook tenderloin until an instant-read thermometer inserted into center of beef reads 125° for medium rare, about 20 minutes.
4. Remove tenderloin from oven and let rest for an additional 20 minutes. During this resting period, tenderloin will continue to rise in temperature to produce a perfect medium-rare interior. To serve, remove twine and use a carving knife to cut beef into ½ inch-thick slices. Transfer slices to a large serving platter, and pour any accumulated juices from cutting board over meat to moisten it. Serve immediately or at room temperature.