Place beef tenderloin on a sheet of aluminum foil, and rub all over with 2 tbsp. oil, the rosemary, and the garlic. Season the tenderloin generously with salt and pepper, and then let the beef sit at room temperature for 1 hour, to allow the seasonings to penetrate the meat and form a flavorful crust on the exterior of the roast. (This resting period also takes the chill off the meat, which allows it to cook more evenly in the oven.)
Heat oven to 425°. Heat the remaining oil and the butter in a 12″ skillet over medium-high heat. Carefully add the tenderloin to the skillet and cook, turning as needed until the beef is browned on all sides, about 7 minutes.
Transfer the skillet to the oven, and cook the tenderloin until an instant-read thermometer inserted into the thickest part of the beef reads 125° for medium-rare.
Remove the tenderloin from the oven and let it rest for 20 minutes. (During this resting period, the tenderloin will continue to rise in temperature to produce a perfect medium-rare interior, and the tenderloin's juices will have enough time to distribute evenly throughout the meat.)
When you're ready to serve the tenderloin, remove the twine, and use a long slicing knife to cut the beef into 1⁄2″-thick slices. Transfer slices to a large serving platter, and pour any accumulated juices from the cutting board over the meat to moisten it. Serve immediately or at room temperature with roasted potatoes and a green salad, if you like.