In a large heavy frypan over medium heat, cook lamb until evenly brown; drain excess fat. Remove from heat and stir in onion, beef stock and Worcestershire sauce. Season with salt and pepper.
On a lightly floured surface, roll pastry out to 3mm thickness. Cut into 15cm rounds. Place approximately 1/2 cup filling on one half of each. Fold the pastry over the filling and crimp edges to seal. Brush lightly with beaten egg white and cut three slits in the top to allow steam to escape. Place on baking sheet.
Bake in preheated oven for 30 to 35 minutes, or until golden brown.