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Preheat the oven to 400°. On a lightly floured sheet of parchment paper, roll out the puff pastry to a 7 1/2-by-15-inch rectangle, about 1/8 inch thick. Cut the pastry into 1 1/2-inch squares.
Spoon a scant 1/4 teaspoon of the Boursin in the center of each pastry square. Season the beef with salt and pepper and set a cube on each pastry square. Fold the pastry over the beef, neatly tucking in the corners. Arrange the pastries seam side down on a large baking sheet lined with parchment paper. The recipe can be prepared to this point and frozen for up to 1 week.
Lightly brush each pastry with the beaten egg. Bake for 10 to 12 minutes, or until puffed and golden brown. Let cool slightly, then transfer to a platter and serve.