For the salmoriglio: In a medium bowl, whisk together the garlic, rosemary, thyme, lemon zest, lemon juice, olive oil and salt. Set aside at room temperature for at least 45 minutes or in the refrigerator for up to 8 hours to let the flavors marry.
For the steak: Remove the porterhouse from the refrigerator 30 minutes prior to cooking; preheat the oven to 450 degrees F.
In a 12-inch stainless steel or cast-iron skillet, heat the olive oil over high heat for 3 minutes. Sprinkle the steak all over with the salt. When the oil begins to show slight wisps of smoke, add the steak to the pan and cook undisturbed for 2 minutes. Flip the steak and transfer the pan to the oven. Roast for 7 to 10 minutes for medium doneness. Remove from the oven and immediately spoon 2 tablespoons of the salmoriglio over the steak. Let rest for 10 minutes before slicing. Serve with the remaining salmoriglio.