The porterhouse is a composite steak coming from the point where the tenderloin and top loin meet. Basically, an over-sized T-Bone steak the porterhouse is generally cut thicker and has much more of the tenderloin relative to the loin portion. If you remove the bone and cut out the two steaks that make up the porterhouse you will get a tenderloin steak and a top loin (or New York Strip Steak).
So if you are ordering a porterhouse, I hope you're hungry. A good porterhouse is also the perfect steak for two people to share.
When buying a porterhouse steak, look for one cut at least 3/4-inch thick with 1-inch thickness being ideal. Occasionally butchers will sell "thin" cut steaks and these are largely pointless. A porterhouse should be thick and not just because it is a huge steak and deserves to be so. To get a large steak like this cooked to perfection, regardless of how you like it done, without it drying out, it needs to have mass and thickness.
A good porterhouse should have a deep, rich color without any gray. The fat should be white and not yellow. There should be good marbling through the meat, particularly the loin portion. I don't recommend trying to get a bargain on a steak like this. While dry aged and/or prime grade versions of the porterhouse will be expensive, you can find a good "choice" grade steak that is fresh and very flavorful.
Because both the loin and tenderloin are quite forgiving a Porterhouse can be cooked any way you like. It is perfect grilled but can also be broiled, sauteed or pan-fried. Use light seasoning with a good amount of salt and try not to cook it beyond medium-well at the very most.
To grill a porterhouse start hot and fast giving the surface a good sear.
Go for the point right before the fat would start to burn then flip it over on the other side. To get those restaurant-style diamond grill marks rotate it 45 degrees on the same side before flipping and repeating on the other side. Once it is well browned on both sides, move it to a cooler part of the grill to finish off to the desired doneness. To give it an extra richness place a pat of butter in the center of the steak a few minutes before removing from the grill. This is a popular restaurant trick that brings out the flavor.
A porterhouse can be served whole or sliced depending on how you want to divide it up. For the very hearty appetite, it is a tremendous meal, a challenge even. As a steak for two, this makes the perfect "special occasion" steak.