Place a steak on a plate and allow it to come to room temperature for 1 hour. Heat oven to 500°. Open all your windows, readying your kitchen for quite a bit of smoke. Heat a large cast-iron skillet over high heat until very hot, 6-8 minutes.
Rub steak all over with 2 tbsp. of the oil, then season generously on all sides with salt and pepper. Add remaining 1 tbsp.oil to skillet and carefully add steak. Cook, without turning, until deeply browned and crusty. When you're cooking a steak at home, it's nearly impossible to brown both sides sufficiently and still cook it to medium rare, so make sure you cook the first side (the presentation side) until it has a charcoal-like crust, about 7-8 minutes.
Flip steak and cook until browned on second side, 2-3 minutes more. Transfer skillet to oven and roast until medium rare, 6-8 minutes.
Transfer steak to a rack over a tray, top with butter cut into thin slices, and tent loosely with foil. Set steak aside to let rest for 5-10 minutes, then transfer to a cutting board. (Hold on to the buttery juices that have collected on the tray.)
Following the contour of the bones, cut the filet off one side of the steak and the top loin off the other. Working on the bias, slice both pieces of meat into 1⁄2"-thick slices, keeping slices together as you go. Reassemble the steak on a platter along with bone, fanning out steak slices slightly. Pour reserved juices over meat.