Place the pastry sheets onto the bench and allow to defrost.
Meanwhile, heat the olive oil in a frying pan over medium-high heat.
Add the onion and cook until just starting to soften.
Add the cubed beef and cook, stirring occasionally, until browned and the liquid has evaporated (approx 5 minutes).
Add the beef stock, tomato paste and worcestershire sauce and stir.
Simmer uncovered for 45 minutes (or until starting to thicken), stirring occasionally.
Mix the cornflour and water together until completely dissolved.
Stir as you pour the cornflour and water into the beef mixture.
Allow to simmer and thicken (the mixture should be thick).
Season the mixture with salt and pepper and then set aside to cool completely (the mixture will continue to thicken on standing).
Preheat oven to 200 degrees celsius (fan-forced).
Lightly grease a 12 hole muffin tray.
Use a small bowl or cutter (approximately 13cm in diameter) to cut 12 circles from the shortcrust pastry sheets (2 per sheet).
Use your fingers to press them into the muffin tray holes (each pastry round should be large enough to slightly overhang).TIP: Cut a small triangle section out from each pastry circle as this helps when fitting them into the muffin holes. Divide the mixture equally between the 12 muffin tray holes (being sure not to over-fill).
Cut 12 slightly smaller rounds (approximately 10cm in diameter) from the puff pastry sheets (4 per sheet).
Place them over the top of the meat pies and use your fingers to seal the overhanging shortcrust pastry to the puff pastry top.
Lightly brush over the beaten egg.
Sprinkle with sesame seeds (optional).
Bake for 30 minutes or until golden.
Serve immediately or freeze in an airtight container for up to 3 months.