Tie the tenderloin roast with kitchen twine every 2 to 3 inches. (Cinching the meat with twine will give it a more uniform shape and ensure even cooking.) Season with kosher salt and refrigerate for 4 hours. Remove the roast from the refrigerator 30 minutes before cooking.
Preheat the oven or bbq to 400°F.
Combine mushrooms, rosemary, and peppercorns in spice grinder or mortar and pestle. Grind to a coarse powder. Rub roast with 2 teaspoons olive oil, then coat all over with porcini rub.
To a cast iron pan over medium-high heat, add 1 tablespoon olive oil and heat until almost smoking. Sear the roast in the hot skillet for 2 to 3 minutes per side, turning three times to sear on all four sides, 8 to 12 minutes total.
Transfer to the oven to roast for about 20 to 25 minutes, until a thermometer inserted into the thickest part of the meat reads 120°F to 125°F for medium-rare.
Remove the beef from the oven and transfer to a cutting board. Let rest, loosely covered, for about 5 minutes before slicing. The temperature of the beef will continue to rise about another 5 degrees.