01 - Preheat oven to 425°F.
02 - Pat the beef tenderloin dry with paper towels. Rub with olive oil, kosher salt, black pepper, and fresh thyme leaves.
03 - Heat a large oven-safe skillet over medium-high heat. Sear the tenderloin on all sides for 2-3 minutes per side until deeply browned.
04 - Transfer skillet to the oven and roast for 15-20 minutes until internal temperature reaches 130°F for medium-rare. Remove from oven, cover loosely with foil, and rest for 10 minutes.
05 - In a large skillet, melt butter over medium heat. Add shallots and cook for 2 minutes until softened.
06 - Add mushrooms and sauté for 5-7 minutes until golden and moisture evaporates. Stir in garlic and cook for 1 minute.
07 - Pour in white wine or beef broth, scraping up any browned bits from the pan. Simmer until reduced by half, about 3 minutes.
08 - Stir in heavy cream, Dijon mustard, and lemon juice. Simmer gently for 3-5 minutes until thickened. Season with salt and pepper.
09 - Slice the rested tenderloin and serve topped with the creamy mushroom sauce. Garnish with fresh parsley.