Forfar Bridies (Scottish Hand Pies)

INGREDIENTS

1 lb ground beef

1 large onion, finely chopped

1 teaspoon dijon mustard

1 1/2 teaspoons salt

3/4 teaspoon black pepper

2 tablespoons beef or vegetable stock

2 tablespoons unsalted butter, cold and cut into small cubes

1 (16 oz) package puff pastry or 1 (14 oz) package empanada baking discs, thawed

1 egg white

PREPARATION