Let the steak rest at room temperature for 30 minutes.
Preheat the broiler. Heat a cast-iron skillet over medium-high heat until very hot.
Season the steak heavily with salt and pepper on all sides. Add the oil to the skillet, then place the steak in the skillet and do not move it. Cook until a nice sear forms, about 3 minutes. Remove the skillet from the heat, transfer the steak to a cutting board and cut both the filet and sirloin from the bone. Slice the steak against the grain into thick pieces. Put the bone back into the skillet and reassemble the steak around it (it should look like the steak originally did) with the seared side up. Top with pieces of butter. Add a couple cloves of garlic and a few sprigs of thyme or rosemary, if desired.
Broil to desired doneness, about 4 minutes for medium rare, 5 to 6 minutes for medium. Transfer to a serving plate and pour the pan drippings over the steak.