Preheat oven to 400°F. Line a baking sheet with parchment paper.
In a small bowl, combine egg and 2 tsp water.
Remove pastry from fridge. Using a rolling pin, roll out dough to a large rectangle, about ⅛ inch thick.
Using a box grater, grate the remaining ¼ cup (½ stick) cold butter over the dough. Fold it onto itself several times to form a square. Roll out dough again, about ⅛ inch thick.
Using a side plate as your template, trace out circles into the dough with a paring knife. Carefully spoon the cooled beef filling on one half of each disk. Brush edges with egg wash, then fold dough over to create a half moon. Using a fork, gently crimp the edges together. Brush tops with egg wash and place on the prepared baking sheet.
Bake for 25 minutes, until golden. Serve immediately.