How to Make It
Slice each bread round in half horizontally. Divide prosciutto slices between bottom halves of bread, and top each bread half with fontina cheese, arugula, and red onion slices. Drizzle balsamic vinegar over sandwiches, and sprinkle with pepper; cover with top halves of bread. Wrap sandwiches tightly in aluminum foil, and bake at 300° for 15 minutes.