- 1 (6- to 6 1/2-pound) whole beef tenderloin, trimmed of fat, silverskin, and the chain (see Cooks' Notes); or a 5- to 5 1/2-pound well-trimmed tenderloin roast
- Salt
- Freshly ground black pepper
- 4 tablespoons olive oil, divided
- 1/2 ounce (about 1/2 cup) dried porcini mushrooms, ground to a powder in a blender
- 1/4 cup (2 ounces) black-truffle butter, softened
- 1/4 cup all-purpose flour
- 1 pound crimini (baby bella) mushrooms, trimmed and quartered into wedges
- 3 cups beef stock or low-sodium broth, divided
- 1/2 cup chopped scallion greens
- Special equipment: Kitchen string; large heavy roasting pan; instant-read thermometer
- 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.