Preheat the oven to 500 degrees F (260 degrees C).
Make slits all over the roast by pricking with a small knife. Insert slivers of sliced garlic. Season the roast with salt and pepper, then spread generously with mustard. Place on a rack in a roasting pan, and cover.
Roast for 60 minutes in the preheated oven. Turn off oven. Leave oven closed, and do not peek for 90 minutes. The internal temperature of the meat should be at least 140 degrees F (60 degrees C) for medium-rare, or 155 degrees F (68 degrees C) for medium.