In a medium bowl, combine panko, fontina, and sage. Stand roast on end and insert thin-bladed knife down center of loin to make 2-inch opening all the way through. Use your fingers to force the slit into a long 1-inch-diameter hole. Pack panko mixture into hole, working from both ends of roast. Season roast on all sides with salt and pepper.
Place roast, fat side up, on work surface. Arrange prosciutto slices across top of roast. Tie with kitchen twine at 1- to 1 1/2-inch intervals. Place in shallow roasting pan, prosciutto side up, and roast 45 minutes to 1 hour, or until internal temperature reaches 145 degrees F.
Remove roast from oven and let rest 10 minutes. Remove twine. Slice roast and arrange on serving platter. Serve drizzled with any pan juices.