Step 1: Remove the beef tenderloin from the refrigerator and let it sit at room temperature for at least 1 hour before cooking. Pat the meat dry with paper towels to ensure a good sear for the beef tenderloin with mushroom sauce.
Step 2: Preheat your oven to 425°F (220°C). Season the beef generously on all sides with kosher salt and black pepper.
Step 3: In a large oven-proof cast-iron skillet, heat the olive oil over high heat until it begins to shimmer.
Step 4: Place the beef in the skillet and sear for 3-4 minutes per side, including the ends, until a deep brown crust forms everywhere.
Step 5: Transfer the skillet to the preheated oven and roast until the internal temperature reaches 130°F (54°C) for medium-rare, typically 20-25 minutes.
Step 6: Remove the beef from the skillet and place it on a cutting board. Tent loosely with foil and let it rest for at least 15-20 minutes.
Step 7: While the beef rests, prepare the mushroom sauce in the same skillet. Melt 2 tablespoons of butter over medium heat. Add the mushrooms and cook until they are browned and the moisture has evaporated.
Step 8: Stir in the shallots and garlic, cooking for another 2 minutes until fragrant.
Step 9: Pour in the red wine to deglaze the pan, scraping up all the brown bits (fond) from the bottom. Let the wine reduce by half.
Step 10: Add the beef stock and heavy cream. Simmer for 5-7 minutes until the sauce thickens enough to coat the back of a spoon. Stir in the fresh thyme and remaining butter.
Step 11: Slice the beef into thick steaks and serve immediately, drizzled generously with the mushroom sauce.