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1
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.
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2
Heat oven to 350u00b0F with rack in middle.
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3
Pat tenderloin dry and sprinkle all over with 1 teaspoon each salt and pepper, rubbing into meat.
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4
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.
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5
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 120u00b0F, 30 to 40 minutes.
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6
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 saute 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.
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7
Add 2 cups stock to skillet and simmer, stirring up brown bits, 1 minute. Remove skillet from heat.
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8
Transfer tenderloin to a cutting board and let stand 15 minutes (internal temperature will rise to about 130u00b0F for medium-rare).
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9
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.
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10
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.
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11
Cut tenderloin into 1/2-inch-thick slices, discarding string, and serve with sauce.
