Prepare your shortcrust pastry (homemade or ready-made) and layer it in your baking dish. Blind bake the pastry until golden brown.
Cook the cubed beef over a high heat in batches until browned (approx. 5 minutes). Set browned beef cubes aside in a separate bowl. Leave the excess fat in the frying pan.
Place the onion, mushrooms and bacon into the frying pan and fry over medium-high heat until softened and the liquid has evaporated.
Add the cooked beef, beef stock, tomato paste and Worcestershire sauce and stir. Simmer uncovered for 45 minutes (or until starting to thicken), stirring occasionally. Add the cornflour mixture whilst stirring and simmer until thickened.
Spoon the cooled pie filling into the prepared shortcrust pastry base.
Place the sheet of thawed puff pastry over the top of the pie and press the edges of the puff pastry together with the shortcrust pastry base. Lightly brush the top of the pastry with the beaten egg. Sprinkle with sesame seeds and then bake until the pastry is puffed, flaky and golden.