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.
Spread meat with mustard. Lay basil and sage leaves on mustard. Sprinkle with thyme leaves, garlic, and pepper.
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.
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.
Transfer meat to a platter. Remove strings. Garnish with herb sprigs. Slice and add salt and pepper to taste.