Preheat oven to 450°F (230°C).

Prepare the filling: In a large bowl, combine shredded roast beef, cubed potatoes, salt, and pepper.

Prepare the pastry: Roll out the pie crusts and cut them in half to form four semi-circles.

Fill the pasties: Spoon a generous amount of the beef and potato mixture onto one side of each pastry. Fold over and seal the edges by pressing with a fork.

Vent and bake: Make small slits in the top of each pasty for steam to escape. Brush the tops with the beaten egg for a golden finish.

Bake for 15-20 minutes, or until the crust is golden brown. Remove from the oven and brush with melted butter if desired.

Prepare the gravy: In a saucepan, bring beef broth or roast drippings to a boil. Stir in beef bouillon. Slowly add the cornstarch mixture, stirring constantly until thickened. Season with salt and pepper.

Serve the pasties warm with the beef gravy on the side or drizzled over the top.