The top round comes from the inside of a leg of lamb and is very tender. Because of its size, it does not take long to cook and is perfect for grilling.
Heat grill to medium-high. Rinse lamb, and pat dry with paper towels. In a shallow dish, whisk together oil and mustard. Stir in mint and salt; season with pepper. Place lamb in dish, and rub mixture evenly over meat.
Place lamb on grill, and cover grill (to help the inside of the lamb cook before the outside begins to char). Grill, turning the lamb as it cooks, until it is dark brown outside and pink inside, about 12 minutes per side for medium-rare, depending on heat of grill. The meat should register 123 degrees on an instant-read thermometer (for medium-rare).
Transfer the lamb to a carving board, and let rest 15 minutes before slicing. Transfer lamb to a large serving platter, and garnish with mint sprigs and grape tomatoes, if desired.