Remove the roast from the fridge 2-3 hours prior to roasting so it comes to room temp. This is a very important step, don’t skip it!
Preheat the oven to 210°F/100°C.
Pat the roast dry with paper towels, if it wasn’t salted before then generously season it with kosher salt, and sprinkle with ground black pepper from all sides.
Make the rub. In a small bowl combine the olive oil with rosemary and garlic then rub on the roast, and spread it well with your hands.
Place the roast on a roasting pan with a rack, fat cap side UP and place in the oven until the internal temperature registers 125-130°F/51-54°C for medium rare. This will take around 2 and half to 4 and half hours depending on the size of the roast. Start checking the internal temperature at 2 hours using a meat thermometer, to make sure that you don't end up overcooking the roast.
Remove from the oven onto a cutting board, cover loosely with foil and allow to rest for one hour.
Preheat the oven to the highest possible temp (mine goes up to 500°F/250°C), remove foil and place the roast on the roaster again and “sear” in the oven for 8-10 minutes. This will form a nice crust that is brown and crisp. Remove from the oven, no need to let it rest, carve and serve with horseradish sauce.