Forfar Bridies

Preheat oven to 440˚F. Combine beef, onion, mustard, salt, pepper, and stock in a large bowl. Fold in butter.
If using puff pastry cut into 6-8 equal circles about 6″ across. Add filling to circles and wet one half of the perimeter. Fold over and press with a fork to seal the edges. Brush with beaten egg white and cut a couple slits in the top of each one for steam to come out.
Bake for 32-35 minutes or until golden brown on outside, covering with foil about halfway through baking. Allow to cool for 5 minutes before serving.