1. Bring meat to room temperature, about 1 hour. Cut through meat horizontally (parallel to work surface) to within 1/2 in. of other side. Lay meat open like a book and spread with mustard, then top with basil and sage leaves. Sprinkle with thyme leaves, garlic, 1 tsp. salt, and 1/4 tsp. pepper.
2. Close up roast and tie with cotton string at 1-in. intervals. Rub all over with oil and sprinkle with 1 tsp. salt and 1/4 tsp. pepper.
3. Prepare a charcoal or gas grill for indirect medium heat, using a drip pan filled halfway with warm water. Grill beef over indirect-heat area, covered, until an instant-read thermometer inserted into thickest part registers 130° to 135° for rare, 60 to 70 minutes. Transfer beef to a platter and let rest 20 minutes.
4. Remove string. Top beef with herb sprigs. Slice and add salt and pepper to taste.