Preheat oven to 220 C / Gas 7. Butter a baking tray, or line with baking parchment.
For the filling, peel the apples and cut into quarters, then cut into thin slices. In a large bowl, drizzle with the lemon juice to prevent discolouring.
Mix the sugar, cinnamon and flour in a small bowl. Add the flour mixture to the apples with the vanilla. Stir everything well. Now add the raisins.
Roll out the puff pastry. Place the filling down the centre. If there is too much fulling, don’t use it all.
Fold the top flap of pastry over the filling and brush with the egg-milk mixture. Fold the lower flap of pastry over it and press firmly. Seal the ends equally well with the egg-milk mixture.
Turn the strudel over so that the nice side is facing up. Lay the strudel on the buttered baking tray and brush it with the remaining egg-milk mixture. Pierce the top a few times with a knife so the steam can escape (diagonal slits look nice).
Bake the strudel in the middle of the oven for 20 minutes. Reduce the temperature to 190 C / Gas 5 and bake for another 20 minutes until golden brown.
Leave to cool for at least 20 minutes before cutting. Dust with icing sugar. A large dollop of cream, custard or vanilla ice cream goes really well with it.