On a floured work surface, roll out puff pastry to a 10 1/2x15" rectangle, halve lengthwise.
Top the halves with the prosciutto and Fontina, leaving a 1/2" 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.
Line 2 large baking sheets with parchment paper. Working with one log at a time, trim the eneven ends. Cut the log crosswise into 1/4" slices. Set the pinwheels 2" apart on the baking sheets. Freeze until firm. Cut the remaining log, then freeze. The frozen slices can be transferred to a resealable plastic freezer bag and frozen for up to one month.
Preheat oven to 375 degrees.
Bake the frozen pinwheels for 20 minutes on parchment-lined baking sheets for 20 minutes, until golden.