A T-bone steak would also work in this recipe, but take into account whether the cut is smaller than the standard porterhouse and adjust the cooking time as needed.
Prepare marinade: Combine rosemary sprigs, garlic, and 1 tablespoon olive oil in a shallow dish or plastic storage bag large enough to hold the steak. Add steak, and season with pepper. Turn to coat, and reseason. Cover with plastic wrap, and allow to marinate overnight in the refrigerator. Meanwhile, prepare compound butter.
Preheat oven to 375 degrees. Heat cast-iron skillet over medium heat. Add remaining 2 tablespoons olive oil and butter. When hot, add steak and a few sprigs of rosemary and garlic from marinade. Cook for about 2 to 3 minutes on each side, being careful not to burn garlic. Lightly season with salt. Transfer to oven for 25 to 30 minutes for medium-rare, turning every 7 to 10 minutes.
When done, transfer steak to a cutting board, and let rest 5 to 7 minutes before slicing. While steak rests, remove garlic and rosemary from skillet, and heat over medium heat. Add wine, and reduce by about one-third. Add compound butter, and stir until just melted. Remove sauce from heat. Cut meat from both sides of the bone, then slice at an angle into pieces about 1/2 inch thick; do not puncture the meat with the carving fork or you will lose juices. Serve with warm sauce drizzled over the meat.