1. Trim any excess fat from meat and discard. Cut through tenderloin lengthwise to within 1/2 inch of other side. Lay meat open like a book.
2. Spread meat with mustard. Lay basil and sage leaves on mustard. Sprinkle with thyme leaves, garlic, and pepper.
3. Bring cut sides together and tie roast with cotton string at about 1-inch intervals to secure. Coat surface of beef with butter and sprinkle with more pepper. Lay roast, cut to the side, on a rack in a 12- by 17-inch pan.
4. Bake in a 425° oven until a thermometer inserted in center of thickest part registers 130° to 135° for rare, 40 to 50 minutes. Let rest in a warm place up to 20 minutes.
5. Transfer meat to a platter. Remove strings. Garnish with herb sprigs. Slice and add salt and pepper to taste.