The Porterhouse is cut from the short loin nearest the sirloin and is popular because of its generous section of tenderloin. It usually is cut to be 1 1/4 to 3 inches thick. One good-sized steak easily feeds four. Porterhouses are distinguished by their "tail," which should be folded back against the steak and held in place with a small metal or wooden skewer for even grilling. The tail can also be removed, ground, and cooked alongside the steak as a burger. When grilling any steak, but particularly a Porterhouse steak with a tail, watch for flare-ups when the fat drips on the coals. Extinguish them with a spritz of water from a spray bottle.
Trim the outer fat from the steak, or ask the butcher to do so. Rub the fat with the cut lemon to prevent burning and smoking. Score both sides of the tail of the steak and then fold it back toward the main body and attach it to the meat with a small metal or wooden skewer. (If using a wooden skewer, soak it in water for about 20 minutes first.)
Combine the olive oil, garlic, scallions, salt, and pepper in a glass or ceramic dish. Put the steak in the dish and turn several times to coat. Cover and marinate at room temperature for 1 hour or in the refrigerator for as long as 4 hours. Turn the meat once or twice during marinating. Return the meat to room temperature before grilling.
Prepare a charcoal or gas grill. Lightly spray the grill rack with vegetable oil cooking spray. The coals should be moderately hot to hot.
Lift the steak from the marinade. Discard the marinade. Sear the steak for 2 to 3 minutes per side. Continue grilling until reaching the desired degree of doneness. Allow the meat to rest for a few minutes before carving. Transfer to a warmed serving platter.