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.

•Whole tenderloins can come in various stages of trimming, particularly those from big-box stores. Some still have the chain attached—a long, thin, loose muscle hanging onto one side of the tenderloin—which you'll want to remove and save for something like kebabs. (Either ask the butcher to do the honors or do it yourself.) Whether the tenderloin comes trimmed (a.k.a. peeled) or not, make sure there is no fat or silverskin left on the outside. If there is, remove it yourself with a sharp knife.•Porcini mushrooms can be ground 1 week ahead and kept in an airtight container at cool room temperature.•You can find black-truffle butter at specialty foods stores, or order it online from Amazon or D'Artagnan.•Crimini mushroom mixture can be made 1 day ahead and refrigerated once cooled. Reheat before proceeding with recipe.