Preheat oven to 440°F (225°C).

In a large bowl, combine ground beef, onion, mustard, salt, pepper, and stock. Fold in the cubed butter.

If using puff pastry, roll out and cut into 6–8 equal circles, each about 6 inches across.

Place a spoonful of filling onto one half of each circle. Moisten the edge of half the circle with water.

Fold the pastry over the filling to create a half-moon shape and press the edges with a fork to seal.

Brush the tops with the beaten egg white. Use a sharp knife to cut a couple of small slits on top for steam release.

Bake for 32–35 minutes or until golden brown. Cover with foil halfway through baking if browning too quickly.

Let cool for 5 minutes before serving.