Remove the porterhouse steak from the fridge at least an hour before cooking. Season the steak liberally with the salt as soon as it comes out of the fridge.
You can make the Mâitre d’ Hotel Butter several days in advance. Mix together the butter, lemon juice, parsley, and salt in a medium mixing bowl with a spatula. Cut a piece of parchment paper in half so you have something close to a printer paper sheet (8×11). Spoon the butter across the length of a piece of parchment — make sure you leave about 2 inches between the butter and the end of the paper. Roll the parchment over the butter and pull to round the butter into a log. Tightly wrap around the remaining parchment. Chill this mixture for at least 30 minutes.
Heat a broiler to high and move an oven rack to about 6-inch under the broiler. Coat the onion halves in olive oil and place them on a baking sheet and set this on another oven rack under the first. Set the steak on the rack directly under the broiler and broil for 2 to 3 minutes per side or until they have a nice burnished sear. Remove the steak from the broiler and set aside to rest for at least 10 minutes.
Preheat a cast iron skillet under the broiler while the steak rests. After resting, slice the steak off the bone and into finger-thick slices (about 1/2 inch). Return the bone and then the steak pieces to the skillet. Top with a few thick slices of the butter and return to the broiler. Broil for 6 to 8 minutes or until cooked to medium rare.