If making your bechemal, heat a saucepan and add 50g butter. Stir in 50g plain flour and cook for around 5 mins, then add 400ml of milk for 10/15 minutes until the flour is cooked through and the sauce is quite thick.
Preheat oven to 180°c/350°F.
Line a baking tray with greaseproof paper and spray with cooking spray.
Lay your pastry over the sheet.
Spread the bechemal over the surface then lay emmental, ham, mustard and another of layer of emmental.
Add the top sheet and press around the sides to stick the sheets together
Brush the top with the beaten egg.
Bake for 25 minutes (or until golden brown on top).