Remove the beef tenderloin from the refrigerator at least 1-2 hours before cooking to bring it closer to room temperature. Pat it thoroughly dry with paper towels. This is crucial for a good sear.

Season the entire tenderloin generously with 2 tablespoons of olive oil, sea salt, and freshly ground black pepper.

In a small bowl, combine the minced garlic, finely chopped fresh rosemary, finely chopped fresh thyme, and 1 tablespoon of olive oil (and Dijon mustard if using). Mix well to form a paste.

Rub the herb mixture evenly over all surfaces of the seasoned beef tenderloin.

Preheat your oven to 425°F (220°C).

Heat a large, oven-safe cast-iron skillet or heavy-bottomed pan over medium-high heat until very hot. Add a tablespoon of neutral oil (like canola or grapeseed if needed, but the olive oil on the tenderloin might be enough).

Carefully place the tenderloin in the hot skillet and sear on all sides (about 2-3 minutes per side) until a deep brown crust forms. This usually takes about 8-12 minutes total.

Transfer the seared tenderloin (still in the cast-iron skillet, or move it to a roasting pan with a wire rack if your skillet isn't suitable for the oven) to the preheated oven.

Roast for 25-35 minutes, or until a meat thermometer inserted into the thickest part registers your desired doneness: Rare: 120-125°F (49-52°C), Medium-Rare: 125-130°F (52-54°C), Medium: 130-135°F (54-57°C). Remember that the temperature will rise by 5-10°F (3-6°C) while resting.

Once the desired internal temperature is reached, remove the tenderloin from the oven. Transfer it to a cutting board, tent loosely with aluminum foil, and let it rest for at least 15-20 minutes. This allows the juices to redistribute, ensuring a tender and moist roast.

Remove the butcher's twine, slice the tenderloin into thick medallions (about 1/2 to 3/4 inch thick), and serve immediately.