Pre-heat the oven to 220°C (428℉). Line a large oven tray with baking paper.
In a large bowl, mix the grated cheese, cream cheese, Worcestershire sauce, chopped onion and salt and pepper together.
Roll out one of the pastry sheets on a large oven tray.
Spread the cheese mixture evenly over the puff pastry, leaving around a 1½cm (½") edge all the way around.
Beat the egg, and brush over the edge all the way around.
Take the other sheet of pastry and place it on top.
Push down the edges to seal, and then carefully roll over the edges all the way around (see note 2).
Once done, press down, and repeat the procedure again. Crimp with a fork, all the way around (this just ensures that the contents remain inside the pastry).
Make several scores in the pastry with a sharp knife.
Brush the egg all over the top of the pastry, and then sprinkle with poppy seeds (optional).
Bake for around 15 minutes, just until golden brown.
Remove from the oven and allow it to cool for a while.
Cut into any size slices you like (I cut mine into 6 large slices) and enjoy it warm or cold.