Make the butter: In a medium saute pan, heat 2 tablespoons of the butter over medium heat, then add the shallots. Season with salt, to taste, and cook until they are translucent, 3 to 5 minutes. Transfer the shallots to a medium bowl. Add the green peppercorns, mustard, lemon juice, lemon zest and Worcestershire sauce. Whisk to blend so all of the flavors start to meld together. Use a fork to blend in the other 8 tablespoons of butter. Season with salt and pepper, to taste, and stir in the parsley.
Roll the butter into a cylinder (like cookie dough) in plastic wrap or parchment pepper and refrigerate until ready to use.
Cook the steak: Heat a cast iron skillet large enough to hold the steak until it begins to visibly smoke. Use a kitchen towel to "blot" any excess moisture from both sides of the steak and season with salt and pepper. Shut the heat off underneath the skillet and use a pair of tongs to put the steak squarely in the pan. Raise the heat high and brown on the first side, 3 to 5 minutes. Resist the temptation to move it as it cooks. Lower the heat and cook for an additional 8 to 10 minutes. Turn it on its second side and brown for 3 to 5 minutes. Lower the heat and cook for an additional 8 to 10 minutes. Turn the steak on the edge with the fat "cap" and brown that for 3 to 5 minutes. Lower the heat and allow the steak to cook for an additional 3 to 5 minutes on each side. Slice the maitre d' butter into1/2-inch rounds and set aside.
Testing for doneness: The simplest way to check for doneness is to make a small incision by the center bone in the thickest part of the steak. It should be a little less cooked than you would like to allow for "carry over" cooking and the "finishing" touch of running the steak under the broiler. For rare, a 2 1/2-inch porterhouse steak should take around 30 to 35 minutes to cook. Add about 4 minutes for medium-rare and 4 additional for medium. If using an instant-read thermometer, rare registers between 125 and 130 degrees F. For medium-rare, 130 to 135 degrees F and between 135 and 140 degrees F for medium.
Remove the steak from the pan to a cutting board and let rest for 10 minutes. Cut into slices and transfer the whole thing to a heatproof platter with the meat still arranged around each side of the center bone. Top the steak with a single layer of the butter slices, put it under the broiler until the butter softens, about 30 to 45 seconds. Serve immediately.
Recipe courtesy of Alex Guarnaschelli