Remove the beef tenderloin from the refrigerator 1-2 hours before cooking to bring it closer to room temperature. Pat it thoroughly dry with paper towels. Trim any silver skin and excess fat. If your tenderloin has a thin "tail" end, fold it under and tie the tenderloin with butcher's twine at 1.5 to 2-inch intervals to ensure even cooking. Season the entire tenderloin generously with 1 tsp salt and 1/2 tsp black pepper.

In a small mixing bowl, combine the finely chopped fresh parsley, rosemary, thyme, minced garlic, Dijon mustard, 2 tbsp olive oil, 1 tsp salt, and 1/2 tsp black pepper. Mix well to form a thick paste.

Evenly spread the herb mixture over the entire surface of the seasoned beef tenderloin, pressing gently to adhere.

Preheat your oven to 400°F (200°C). Heat 1 tbsp olive oil in a large oven-safe cast iron skillet over high heat until shimmering and almost smoking. Carefully place the tenderloin in the hot skillet and sear for 2-3 minutes on all sides until a deep golden-brown crust forms. This step is crucial for flavor and texture.

Transfer the skillet with the seared tenderloin to the preheated oven. If you don't have an oven-safe skillet, transfer the tenderloin to a roasting pan fitted with a rack. Roast for 30-45 minutes, or until an instant-read meat thermometer inserted into the thickest part reads 125-130°F (52-54°C) for medium-rare. For medium, aim for 130-135°F (54-57°C).

Once desired doneness is reached, remove the tenderloin from the oven and transfer it to a clean cutting board. Tent loosely with aluminum foil and let it rest for at least 10-15 minutes. This allows the juices to redistribute, ensuring a tender and moist roast.

Remove the butcher's twine. Slice the tenderloin into 1/2 to 3/4-inch thick medallions against the grain. Serve immediately with your favorite sides.