Preheat oven to 375 degrees.
In a 9x13 dish, lay out one of the sheets of pastry.
Stretch it out so it fix the dish. Spread the cheese filling evenly over the pastry.
Top with second pastry sheet.
Cut into 2 inch squares. Mix the egg and milk together and pour over the top of the casserole.
Bake 30 minutes.
Pastry should be puffed and golden in color.
The pastry will collapse when taken out of the oven, this is normal.
Recut and serve hot.