Step 1: Prepare the Beef. Remove the beef tenderloin from the refrigerator about 30-60 minutes before cooking to allow it to come to room temperature. Pat the beef very dry with paper towels – this is crucial for a good sear. Season generously all over with salt and freshly ground black pepper.
Step 2: Sear the Tenderloin. Heat a heavy-bottomed, oven-safe skillet (cast iron works beautifully) over medium-high heat. Add 1 tablespoon of olive oil. Once shimmering, carefully place the seasoned beef tenderloin in the hot skillet. Sear undisturbed for 3-4 minutes on each side until a beautiful, deep brown crust forms. Use tongs to sear all sides, including the ends.
Step 3: Finish in the Oven. While the beef is searing, preheat your oven to 400°F (200°C). Once seared on all sides, transfer the skillet directly to the preheated oven. Roast for 8-15 minutes, depending on the thickness of your tenderloin and your desired doneness. For medium-rare, aim for an internal temperature of 130-135°F (54-57°C). For medium, 135-140°F (57-60°C). Use an instant-read thermometer inserted into the thickest part of the beef.
Step 4: Rest the Beef. Once the desired temperature is reached, remove the skillet from the oven. Transfer the beef tenderloin to a cutting board, tent it loosely with foil, and let it rest for at least 10-15 minutes. This allows the juices to redistribute, ensuring a tender and juicy result. Don't skip this step!
Step 5: Sauté the Mushrooms. While the beef is resting, return the same skillet (no need to clean it; the fond adds flavor!) to medium heat. Add the remaining 1 tablespoon of olive oil and the 2 tbsp unsalted butter. Add the sliced mushrooms and sauté for 5-7 minutes, stirring occasionally, until they are golden brown and have released their moisture.
Step 6: Build the Sauce Base. Add the minced shallots and garlic to the skillet with the mushrooms. Sauté for another 2-3 minutes until fragrant. Deglaze the pan with the dry white wine (or extra beef broth), scraping up any browned bits from the bottom of the pan. Let it simmer for 1-2 minutes until the liquid is mostly reduced.
Step 7: Create the Creamy Sauce. Pour in the beef broth and heavy cream. Bring the mixture to a gentle simmer and cook for 5-7 minutes, stirring occasionally, until the sauce thickens slightly. Stir in the fresh thyme and Dijon mustard. Season the sauce with salt and black pepper to taste. If you prefer a thicker sauce, you can make a quick slurry of 1 teaspoon cornstarch mixed with 1 tablespoon cold water and whisk it into the simmering sauce.
Step 8: Serve Your Masterpiece. Slice the rested beef tenderloin into thick medallions. Arrange the slices on plates and generously spoon the warm, creamy mushroom sauce over the top. Garnish with fresh parsley or chives, if desired. Serve immediately.