Cut the beef tenderloin in half crosswise. Pat dry with paper towels. If you have time, place the beef tenderloin in the fridge, uncovered, for 8-12 hours or overnight. (This is not absolutely necessary, but helps create a better outer crust when searing, so do it if you can.)

Tuck the long, thin end underneath. Use kitchen twine to tie each piece of tenderloin tightly, at one-inch intervals – see post above for instructions and pictures for how to tie it.

Pat the beef with paper towels again. Season all over with salt and pepper. Let it come to room temperature on the counter for 1 hour before cooking.

Preheat the oven to 400 degrees F (204 degrees C).

In a small bowl, mash together the butter, garlic, rosemary, thyme, and Dijon mustard (if using). Set aside.

Heat the olive oil in a large cast iron skillet over medium-high heat, until shimmering and very hot. Add the beef tenderloin pieces, without touching each other, and sear on all four sides, about 2-3 minutes per side, until a brown crust forms. (If the beef doesn’t want to stay on one of the sides, use tongs to hold it against the pan.) Sear for slightly less time on the last side, as it will continue to sear while you spread on the butter in the next step.

Working quickly, immediately spread the butter mixture over the beef, insert a probe thermometer into the thickest part in the center, and transfer to the preheated oven. (Alternatively, if searing in a pan that is not oven-safe, transfer to a roasting pan before placing in the oven.)

Roast beef tenderloin in the oven until it reaches your desired internal temperature: 115 degrees F for rare (20-30 minutes), 125 degrees F for medium rare (25-35 minutes), 135 degrees F for medium (30-40 minutes), 145 degrees F for medium well (35-45 minutes), or 155 degrees F for well done (40-50 minutes). (Temperature will continue to rise while resting in the next step.) For the juiciest beef, medium rare to medium is recommended.

To prevent overcooking, immediately transfer the roasted beef tenderloin to a cutting board after removing from the oven. Let it rest for 10-15 minutes to allow the juices to settle – the internal temperature will rise an additional 5-10 degrees.