Let steak rest at room temperature for 30 minutes
Preheat broiler, heat cast iron skillet on stovetop over medium-high heat until hot
Season steak with salt and pepper on all sides
Add oil to skillet then place steak in skillet
Cook until you have a nice sear on both sides, about 3 minutes per side
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.Top with pieces of butter. Add a couple cloves of garlic and a few sprigs of thyme or rosemary, if desired.
Broil until desired doneness, about 4 minutes for medium
Transfer to serving plate and pour the pan drippings over steak