In a bowl, combine the pork, onion, potato, parsley, sage, salt, pepper and pepper flakes. Mix with a fork to combine.
Transfer the mixture to a large pastry bag fitted with a large plain tip.
Unfold the puff pastry sheets and, following the fold marks, cut each sheet into three rectangles. Roll out slightly.
Pipe the sausage mixture along one long edge of each of the pastry rectangles. Paint the opposite edges with some of the beaten egg. Starting at the edge with the sausage mixture, roll up each rectangle, gently pressing the egg-painted edge to seal.
Using a sharp knife, cut the rolls into slices about 1 1/2 inches thick. Arrange them on a broiler pan or a wire rack set on top of a baking sheet. Brush the slices with the remaining beaten egg and bake for 20 minutes, or until golden.
Serve the slices warm with the Worcestershire sauce on the side for dipping.