
Remove the trimmed and twine-wrapped beef tenderloin from the fridge. Pat dry with paper towels, cut into two even pieces, and season with kosher salt and pepper. Place on a wire rack on top of a sheet pan to bring it to room temperature before cooking (about 2 hours, depending on thickness).
Combine the butter, garlic, thyme, and rosemary in a small bowl. Set aside.
When ready to cook, preheat the oven to 425°F.
Heat the oil in a large cast-iron skillet or oven-safe pan over medium-high heat. Once shimmering and the pan is very hot, add the beef tenderloin and sear each side for 3–4 minutes, about 12 minutes total, to create a golden brown crust.
Slather the butter herb mixture over the top of the beef tenderloin, then transfer the entire pan to the preheated oven. Roast for about 20 minutes for medium rare, or until an instant-read thermometer reads between 125-130°F. The meat temperature will continue to rise as it rests.
Remove the beef tenderloin from the oven and transfer it to a cutting board. Let it rest for 15 minutes. While the tenderloin rests, prepare the gravy. Remove the twine and slice into 1-inch thick pieces.
Transfer the beef drippings to a heat-safe bowl and return about 4 tablespoons of the fat that rises to the top back to the pan, reserving the remaining drippings. Heat the same skillet over medium heat. Add the shallot, carrot, and celery and cook for 10 minutes, or until the vegetables start to brown. Remove and discard the vegetables with a slotted spoon, taking care to reserve as much of the butter in the pan as possible.
Add the flour to the butter and whisk until it forms a thick paste. Cook, stirring constantly with a wooden spoon for 1–2 minutes, or until the roux turns golden brown.
Whisk in the beef stock, reserved beef drippings, soy sauce, and Worcestershire sauce. Increase the heat to medium-high and bring to a boil. Reduce the heat to a simmer, add the thyme, and cook for 10 minutes, or until it thickens slightly. Taste the gravy and season with salt and pepper as needed. Discard the thyme and serve warm.