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.
