Preparation

Dry-age the beef three to seven days ahead:

• Unwrap the beef, rinse it well, and pat it dry with paper towels. Do not trim. Wrap the roast loosely in a triple layer of cheesecloth and set it on a rack over a rimmed baking sheet or other tray. Refrigerate for three to seven days; the longer the beef ages, the tastier it gets. After the first day, carefully unwrap and then rewrap with the same cheesecloth to keep the cloth fibers from sticking to the meat.
• When ready to roast, unwrap the meat and, with a sharp knife, shave off and discard the hard, dried, outer layer of the meat. Shave away any dried areas of fat, too, but leave behind as much of the good fat as possible.

Roast the beef:

• 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 the 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 375°F. Continue to roast until a thermometer inserted in the center of the roast registers 130°F for medium rare, 1 to 1-1/2 hours.
• Let the beef rest for 20 minutes. Meanwhile, transfer the crème fraîche to a small serving dish. Carve the beef into thin or thick slices and pass the crème fraîche on the side.