Mince the garlic cloves with a chef’s knife and sprinkle with the salt. Using the side of the knife, scrape and mash the garlic and salt together until they turn into a paste.
In a small bowl, combine the garlic paste with mustard, thyme, olive oil, and pepper. Rub the garlic mixture over all sides of the beef. Put the roast, fat side up, on a rack set in a heavy-duty rimmed baking sheet or small roasting pan.
Let the roast sit at room temperature for 1 hour.
Meanwhile, position a rack in the center of the oven and heat the oven to 450°F.
Roast the beef for 15 minutes. Without opening the door, reduce the oven temperature to 350°F. Continue to roast until a thermometer inserted in the center of the roast registers 125°F for medium rare.
Let the beef rest for 20 minutes. Carve beef into desired thickness.