Let your meat sit out at room temperature for an hour prior to roasting. Preheat the oven to 275 degrees F, then place the loin on a sheet pan.
Rub all over with olive oil, then rub generously on all sides with kosher salt and freshly cracked black pepper. Insert a probe thermometer into the center of the meat.
Roast on the center rack of the oven until your desired internal temperature is reached. Rare: 115-120 degrees F. Medium-Rare: 125-130 degrees F (this is what I recommend). Medium: 135-140 degrees F.
Take out the probe, then move the pan up to the top rack of the oven and turn on the broiler. Keep a close watch until it's nicely browned on top, about 30 seconds to a minute, then use a pair of tongs to turn it ¼ of the way. Repeat until browned on all 4 sides.
Remove from the oven, cover with foil, and let rest for at least 15-20 minutes or longer.
Slice and serve as desired.