Pancetta Studded Beff Tenderloin

Roast beef:

Preheat oven to 425°F.

Pat beef dry and cut 1/2-inch-deep slits at 1-inch intervals all over roast, then insert 1 piece of pancetta into each slit. Sprinkle beef with kosher salt and pepper. Heat oil in a 12-inch heavy skillet over high heat until just smoking, then brown beef on all sides, about 5 minutes. Transfer beef to a small roasting pan, reserving skillet.

Roast beef in middle of oven until an instant-read thermometer inserted diagonally 2 inches into center registers 120°F, about 25 minutes. Transfer beef to a cutting board and let stand, loosely covered with foil, 25 minutes. (Beef will continue to cook as it stands, reaching 130°F for medium-rare.)

Make sauce while beef stands:

Heat reserved skillet over moderately high heat until oil is hot but not smoking, then saut é shallot until golden, about 2 minutes. Add red wine and Marsala and deglaze skillet by boiling, stirring and scraping up brown bits, until liquid is reduced by about one third. Add demiglace and jelly and briskly simmer, whisking, until jelly is incorporated, about 2 minutes. Add butter, 1 bit at a time, whisking until incorporated, then remove from heat. Whisk in salt and pepper. Pour sauce through a fine sieve into a bowl, pressing on solids.