Preheat oven to 325 degrees Fahrenheit. Using a small sharp knife, cut small slits in roast, spacing them evenly over surface. Insert garlic slivers into slits. Rub roast with mustard; sprinkle with thyme and pepper.

Place the meat on roasting rack in a large roasting pan. Roast until an instant-read meat thermometer inserted into the thickest part of the meat registers 150 degrees fahrenheit. for medium, about 1 hour and 15 minutes.

Transfer meat to a carving board and let stand for 10 minutes. Remove the string; garnish with thyme sprigs. Carve into thin slices and serve.