A porterhouse is essentially a big T-bone (a cut with the bone in, comprising both sirloin and fillet) but it’s an important part of the dish to have a steak thick enough to get gnarly on the outside while staying rare within. Ask your butcher to ensure it’s 5-7cm thick. Any thinner and it won’t have the right texture (and it will be tricky to stand it up on its side, which is an integral part of the cooking process).