Prep the beef:
Trim excess fat or silverskin if needed. Cut the tenderloin in half to create two even pieces. Tuck the thin ends underneath and tie with butcher’s twine to form an even shape. Season all sides with salt and black pepper.

Optional overnight step:
For the best crust, place the seasoned beef uncovered on a rack in the refrigerator for at least 10 hours or overnight.

Bring to room temperature:
Remove the beef from the fridge and let it rest at room temperature for about 2 hours before cooking.

Sear the tenderloin:
Preheat the oven to 425°F (220°C). Heat avocado oil in a large cast-iron skillet over medium-high heat. Sear the beef on all sides for 3–5 minutes per side until deeply browned.

Make the herb butter:
In a small bowl, mix butter, garlic, horseradish or Dijon, and rosemary or thyme until well combined.

Butter and roast:
Remove the skillet from heat and generously spread the herb butter over the seared beef. Insert a probe thermometer into the thickest part of the meat. Transfer the skillet to the oven and roast for 20–25 minutes, or until the internal temperature reaches 125–130°F for medium-rare.

Rest and slice:
Transfer the beef to a cutting board and let it rest for 15–20 minutes. Remove twine, slice into 1-inch pieces, and serve.