Cut the pastry into two sections, one slightly smaller than the other, roll into rectangles. One 12x20cm the other 17x25cm and lay on a baking sheet. Prick the smaller pastry with a fork and chill.
To make the filling, peel, core and dice the apple into large chunks. In a pan melt the butter and sugar, adding the apples, cinnamon and lemon juice. Heat and cook until the apples are soft but not a puree. Add the Calvados and allow to cool- then add the raisins.
Bake the smaller pastry sheet until brown. Remove from the oven and turn the pastry over and place on a baking sheet. Add the apple topping to the cooked pastry.
Take the uncooked pastry and fold in half lengthways, starting 2cm in from one end cut through the folded side of the pastry a number of parallel lines an even distance apart, leaving a 2cm margin around the three other edges. When you unfold the pastry you will have a clearly defined border around the cuts to the edge of the pastry.
Lay the pastry on top of the apple and wrap the edges underneath the cooked pastry sheet. Brush the top sheet with the milk and sprinkle well with the caster sugar.
Bake for a further 20 minutes. You can serve this warm or cold with cream, crème fraiche or whatever you fancy.
Preparation time – 1 hour
Cooking time – 30-40 minutes