Preheat oven to 400˚F (200˚C). Line two large baking sheets with parchment or lightly grease.
In a large bowl, combine the ground beef, onion, garlic, bread crumbs, parsley, salt, nutmeg, black pepper, and egg just until blended.
Divide the mixture into 8 equal sections and set aside.
In a small bowl, whisk together the egg yolk and milk to make the egg wash.
Place 1 sheet of the puff pastry on a lightly floured surface and roll gently to make an even rectangle without any creases.
Use a pastry cutter or sharp knife to cut the pastry into four equal rectangles about 5 x 6 inches (13 x 15 centimeters) each.
Spread one section of ground beef mixture down the center of one of the puff pastry rectangles.
Brush one long side with the egg wash and fold the pastry over the filling, long side to long side. Press down the long side, pressing out any air pockets and seal with a fork. Leave the two short ends open with the filling still showing.
Place the pastry on the prepared baking sheet and brush the top with the egg wash. If desired, use a sharp knife to cut slits across the top. Repeat with other puff pastry sheet and filling, placing the pastries on the baking sheet at least 1 1/2 inches (4 centimeters) apart.
Bake in the preheated oven until the meat is cooked through and the pastry is puffed and golden, 20-25 minutes.