Trim the tenderloin if needed, divide it into two pieces, fold the narrow end under, and tie each piece with butcher twine. The roast should look even in thickness, with no thin tail left exposed to overcook.

Coat the beef evenly with the salt and black pepper, then refrigerate it uncovered overnight, or for at least 10 hours. The surface will look drier by cooking time, which helps it brown instead of steaming.

If you did not season ahead, tie and season the tenderloin now, then let it sit on the counter for about 2 hours after refrigeration. Taking off the chill helps avoid a cold center and a thick gray band beneath the crust.

Heat the oven to 425°F. Have the skillet, butter mixture, and thermometer ready before the beef begins searing.

Warm the avocado oil in a large cast-iron skillet over medium-high heat, then brown the tenderloin on every side for 3 to 5 minutes per side. Turn when the surface is a deep golden brown and the skillet smells richly savory rather than sharply raw.

Mix the softened butter, garlic, horseradish, and herbs in a small dish, then spread it generously over the seared tenderloin and insert the probe thermometer. The butter should soften over the hot beef and release a fragrant herbal aroma.

Move the skillet to the oven, or transfer the beef to a baking dish, and roast until the thermometer reaches your desired internal temperature, about 20 to 25 minutes. Judge doneness by the probe, not the clock, since thickness changes the roasting pace.

Transfer the tenderloin to a cutting board and rest it for 15 to 20 minutes before cutting 1-inch slices. A rested roast gives cleaner slices with far less flavorful juice pooling across the board.