Preheat oven to 350 degree F. Trim fat from beef, leaving a thin even layer of fat on the top if possible. Season beef with salt and black pepper. Place beef, fat side up, on a rack in a shallow roasting pan. Insert an oven-going meat thermometer into center of roast. In a small bowl combine pesto, mustard, lemon peel, and garlic; set aside.
For medium rare, roast meat, uncovered, for 1-3/4 to 2 hours or until meat thermometer registers 135 degree F, spreading half the pesto mixture over top and sides of beef for the last 20 minutes of roasting. For medium, roast 2 to 2-1/2 hours or until meat thermometer registers 150 degree F, spreading pesto mixture as above. Continue roasting until desired doneness. Cover and let stand for 15 minutes before carving. Temperature of the beef after standing should be 145 degree F for medium rare or 160 degree F for medium. Serve with remaining pesto mixture. Makes 15 servings.