Porcini-Crusted Beef Tenderloin with Truffle Butter Sauce

Make a deep cut into the top side of tenderloin, about 4 inches from tail end, to facilitate folding the tail end under the roast to give the meat a more uniform thickness. Tie roast at 1-inch intervals with kitchen string. Heat oven to 350 °F with rack in middle. Pat tenderloin dry and sprinkle all over with 1 teaspoon each salt and pepper, rubbing into meat. Set roasting pan over 2 burners, then add 3 tablespoons oil and heat over medium-high heat until shimmering. Brown tenderloin on all sides, about 10 minutes. Sprinkle porcini powder evenly on top of tenderloin and roast in oven until thermometer inserted diagonally 2 inches into center of thickest part of meat registers 120 °F, 30 to 40 minutes. Toward end of roasting tenderloin, stir together truffle butter and flour. Heat remaining tablespoon oil in a 12-inch heavy skillet over medium-high heat until shimmering, and sauté mushrooms with 1/4 teaspoon each salt and pepper until liquid released by the mushrooms evaporates and mushrooms are tender and golden, 10 to 12 minutes. Add 2 cups stock to skillet and simmer, stirring up brown bits, 1 minute. Remove skillet from heat. Transfer tenderloin to a cutting board and let stand 15 minutes (internal temperature will rise to about 130 °F for medium-rare). Set roasting pan over 2 burners and add remaining 1 cup stock. Bring to a boil, scraping up brown bits, then pour stock into mushroom mixture in skillet. Bring mushroom mixture to a boil and whisk in truffle butter mixture, bit by bit. Reduce heat and simmer, whisking occasionally until thickened, about 2 minutes. Stir in scallion greens and salt and pepper to taste. Cut tenderloin into 1/2-inch-thick slices, discarding string, and serve with sauce.