1. Preheat the oven to 415°F. Remove the steak from the fridge 30 minutes prior to cooking, this is to bring the steak to room temperature and ensure your cooking times are more accurate. Season all sides liberally with salt and pepper. This can be done while your steak comes to room temp.
2. Add 1-2 tablespoons of oil to an oven-safe cast iron skillet and turn the heat up high, allowing the skillet to become very hot. Twirl the pan to distribute oil as it heats. Once the oil starts to smoke, place the porterhouse face down and sear undisturbed for 2 minutes. Flip the steak and sear for an additional 2 minutes. This will give your steak a nice seared edge.
3. Transfer your skillet directly to the oven. [WARNING] skillet may be hot, handle it with oven mitts. For rare, bake for 4 minutes. Medium rare, 5-6 minutes. Medium, 6-7 minutes. Medium well, 8-9 minutes. Remember, depending on the size of the steak, the more or less time it will take. This recipe is ideal for a 32-ounce porterhouse, roughly 1.5-2 inches thick.
4. Remove steak from the skillet and place on a cutting board to rest. Now is a good time to prepare the au Poivre sauce. Place the same skillet used to cook the steak over medium heat. The pan drippings and crust add flavor! Add the shallots, butter, and peppercorns (as coarse as possible) to the skillet. Cook until butter melts and shallots start to soften, about 2-3 minutes. Stir occasionally to prevent burning. Add the cognac and cook until mostly evaporated, about 2 minutes. Add cream and bring to a simmer. Cook until thickened enough to coat the back of a spoon. About 1 minute.
5. Cut the porterhouse into 1-inch thick strips, separating from the bone. Serve with sauces.

For the chimichurri sauce

1. Combine all ingredients in a blender or food processor. Pulse until smooth, but still a little gritty. Can be made several hours ahead and stored covered in the fridge.

For the horseradish sauce

1. Combine all ingredients together in a small bowl and mix until combined. Can be made several hours ahead and stored covered in the fridge.