On a floured work surface, roll out the puff pastry to a 10 1/2-by-15-inch rectangle; halve lengthwise. Top the halves with the prosciutto and Fontina, leaving a 1/2-inch border along the far edge of each piece. Season with pepper. Starting at the near edge, tightly roll the halves of pastry into logs. Brush the borders with the egg wash and pinch to seal. Transfer the logs to a large baking sheet and freeze for 30 minutes until firm.