Trim any excess fat off the tenderloin (if not already done by the butcher), cut it in half so you have two pieces and tie with butcher twine every few inches.

Season the tenderloin with the salt and pepper, cover loosely with plastic wrap then leave it in the fridge for at least 10 hours. (This is an optional step but helps season the meat all the way through. 

Remove the roast from the fridge about two hours before cooking. Season it with a little more salt and pepper then let it sit uncovered for at least 2 hours so it can come to room temperature. 

Preheat the oven to 435 degrees F. 

Heat the oil to medium high in an oven safe cast iron skillet on the stovetop.

Sear the two tenderloin pieces one at a time in the hot oil. You just want to brown the roast on all sides, about 3 minutes per side. Set aside

Combine the softened butter with the garlic and dijon mustard.

Slather each seared piece of tenderloin in the garlic butter and place a probe thermometer through the center of each piece (using a probe thermometer is the only way to ensure the meat is cooked to your preferred internal temperature). Set your probe thermometer to alert you when the meat has reached your doneness. Rare - 115 - 120 F, Medium Rare 120 - 125 F, Medium 130 - 135 F. 

Place the prepared tenderloins back in the oven safe skillet or on a sheet pan lined with tin foil and bake in the oven until the probe alerts you that the meat is cooked, about 18 - 25 minutes depending on the thickness of the meat. 

Remove the tenderloins from the oven, place them on a cutting board, tent loosely with foil and allow them to sit for 15 minutes to allow the juices to settle. 

Cut off the twine and slice into 1” pieces. Enjoy with horseradish or alone!