Preparation

Make the sauce reduction:

• Melt 1 Tbs. of the butter in a 3- to 4-quart (preferably 8-inch-wide) saucepan over medium-low heat. Add the shallot and cook until softened, about 5 minutes. Add the port and the thyme sprigs and bring to a boil over high heat. Reduce the heat to maintain a brisk simmer and cook until the port has reduced to a syrupy texture, about 30 minutes—you should have about 1/2 cup including the shallot.
• Meanwhile, bring the chicken broth to a simmer in a small saucepan. Off the heat, add the porcini and let them soak for 15 minutes. With a slotted spoon, transfer the porcini to a small bowl. Strain the soaking liquid through a fine sieve lined with a paper towel or coffee filter.
• Add the soaking liquid and half of the porcini (about 1/3 cup) to the port reduction (save the remaining porcini for the stuffing). Bring the sauce to a boil over high heat. Reduce the heat to maintain a brisk simmer and cook until it has reduced to 1-1/3 cups, about 15 minutes. Strain the sauce through a fine strainer, pressing on the solids. You should have about 1 cup sauce. Set aside until the roast is done.

Make the stuffing:

• Soak the currants in hot water for 10 minutes. Drain and put them in a food processor. Add the reserved soaked porcini. Heat 1 Tbs. of the oil in a 10-inch skillet over medium heat. Pat the chicken liver dry and cook on both sides until browned on the outside and just a little pink inside, 3 to 4 minutes total. Transfer to a plate, let cool slightly, and then add to the food processor. Process until finely chopped.
• Heat the remaining 1 Tbs. oil in the skillet over medium-high heat. Add the mushrooms, salt, and pepper. Cook, stirring occasionally, until they look wilted and shrunken, 2 to 3 minutes. Add the shallot and garlic and continue to cook, stirring, until the mushrooms are tender and beginning to brown, 3 to 4 more minutes. Add the chicken broth and scrape the bottom of the pan with a wooden spoon to loosen any brown bits. If the liquid doesn’t evaporate right away, boil until it does. Remove the pan from the heat and let cool slightly.
• Add the sautéed mushrooms, butter, parsley, Cognac or Armagnac, lemon juice, thyme, and lemon zest to the liver mixture in the food processor. Pulse to form a chunky paste and season to taste with additional salt and pepper.
  
• Scrape the stuffing onto a large piece of plastic wrap and shape it into a log a few inches longer than the roast. Tightly roll the stuffing up in the plastic wrap and twist the ends to form a very tight log. Twisting the ends of the plastic will compress the stuffing so that it’s just a bit shorter than the roast. Freeze until firm, about 2 hours and up to 2 days.

Stuff & cook the roast:

• Trim the silverskin and excess fat from the tenderloin. Butterfly the tenderloin by slicing it lengthwise almost but not completely in half, so that you can open it like a book.
  

• Unwrap the stuffing and center it along one half of the roast. Fold the roast back up to its original shape and tie at 1- to 2-inch intervals with butcher’s twine.
  

• Let the roast sit at room temperature for 1 hour (2 hours if the stuffing is frozen solid). Position a rack in the bottom third of the oven and heat the oven to 500°F. Let the remaining 1-1/2 Tbs. butter for the sauce soften at room temperature.
• Season the roast generously with salt and pepper and put it on a flat rack set in a roasting pan or heavy-duty rimmed baking sheet. Roast for 15 minutes and then reduce the oven temperature to 325°F. Continue to roast the beef until a meat thermometer inserted into the center of the meat (not the stuffing) registers 125°F for rare or 130°F for medium rare, 30 to 45 minutes more. Move the roast to a carving board and let it rest, loosely tented with foil, for 15 minutes.

Finish the sauce:

• While the roast rests, bring the sauce to a simmer in a small saucepan over medium-low heat. Mix the softened butter with the flour to form a paste and whisk it into the sauce. Simmer the sauce to thicken slightly and cook off any raw flour taste, about 3 minutes. Whisk in the vinegar. Season to taste with salt, pepper, and additional vinegar.

Plate & serve:

• Remove the strings from the roast and slice it into 8 to 10 medallions. Put a medallion on each plate and drizzle the sauce around the beef.
  

Make Ahead Tips

The reduced, strained sauce can be made and refrigerated up to 2 days ahead. If not making the stuffing ahead, refrigerate the remaining soaked porcini separately. The roast can be stuffed and refrigerated up to 1 day in advance.

A hearty red wine with developed tannins, a mineral undertone, and bright fruit pairs well with the earthymushroom and port elements of the beef tenderloin. The Waterstone 2004 Cabernet Sauvignon from Napa Valley would be perfect.