Whip Up the Red Wine Sauce: In a medium saucepan, melt 4 tablespoons of butter over medium heat. Add the chopped shallots and cook for about 4 minutes until they're soft and fragrant. Pour in the red wine, 2 ¼ cups of beef broth, fresh thyme sprigs, salt, and pepper. Bring it to a boil, then reduce the heat to medium and let it gently simmer for 20 minutes, or until the liquid has reduced by half. Turn off the heat. (You can make this delightful sauce up to 3 days in advance!)
Prep Your Tenderloin: Rub 2 tablespoons of olive oil all over the beef tenderloin. Use your fingers to thoroughly rub the chopped rosemary and minced garlic into the meat. Season the entire roast generously with salt and pepper. Let the beef sit at room temperature for 1 hour before cooking – this step helps tenderize the meat and infuse flavors.
Sear for a Perfect Crust: Preheat your oven to 425°F. In a large, oven-proof skillet or Dutch oven, heat the remaining 2 tablespoons of olive oil and 2 tablespoons of butter over medium-high heat. Carefully add the tenderloin and sear it for about 7 minutes, turning it until it's beautifully browned on all sides. (It's okay if you need to gently bend the roast to fit your pan!)
Roast to Ideal Doneness: Transfer the skillet (or the roast to a baking pan if your skillet isn't oven-safe) to the preheated oven. Cook until an internal temperature of 125°F is reached for a delicious medium-rare. An instant-read thermometer is your best friend here!
Rest and Finish the Sauce: Remove the roast from the oven and let it rest on a cutting board for at least 15-20 minutes. This resting period is key for juicy, tender meat! While it rests, place the skillet back on the stove over medium heat. Add the remaining ¼ cup of beef broth and use a spatula to scrape up all those flavorful bits from the bottom of the pan. Bring it to a simmer and sprinkle with another ¼ teaspoon of salt. Pour this flavorful broth into your reserved red wine sauce and stir to combine. For an extra smooth finish, you can strain the sauce into a gravy boat.
Slice and Serve: Carefully remove the twine from the beef tenderloin. Slice the roast into ½-inch thick medallions. Arrange them on a serving platter and serve warm, or even at room temperature, with the exquisite red wine sauce on the side. For an extra touch of elegance, sprinkle with a pinch of finishing salt, if desired. Enjoy this show-stopping holiday meal!