In a bowl combine Gruyere, and sage. On a lightly floured surface arrange pastry sheet with short side facing you. Brush the edges with the beaten egg. Arrange half of the prosciutto evenly on top of sheet, avoiding the egg brushed edge and top with half of the Gruyere mixture. Starting with the left side, roll pastry to the center. Then on the right side, roll that side to the center. You should have something resembling a butterfly. Seal the middle of the roll with the beaten egg. Chill seam side down until firm at least 3 hours.

Preheat oven to 400 and lightly grease 2 large baking sheets. Cut logs crosswise into 1/4 – 1/2 inch thick butterflies and arrange cut side down on baking sheet one inch apart. Bake in batches till golden 14 to 16 minutes. Transfer to a rack to cool slightly. serve warm. makes about 40 depending on how thick you cut them.

If you want to freeze these, cut, then put in freezer proof containers with waxed paper between layers.