Preheat the oven to 180 C.
Set the puff pastry sheet on the counter to thaw.
Peel and core the apples and slice them thinly.
Melt the butter over medium-low heat.
Add the apples to the pan along with 2 tablespoons of granulated sugar plus all of the brown sugar, corn starch, cinnamon, and allspice.
Sauté the apples for around 5 minutes, until they're coated with sauce and reduce. Turn off the heat, stir in the raisins, then leave the apples to cool completely.
Mix together the beaten egg and the milk.
Roll out the puff pastry into a rectangular shape. Pile the apple mixture down the center of the pastry, leaving an inch of pastry at the sides, and the top apple-free.
Fold the top and bottom of the pastry over the apple mixture, then fold 1 of the sides over the top, covering half of the apples. Using the egg and milk mixture as a pastry glue, brush the exposed edge of the pastry before folding the other side of the pastry, slightly overlapping the first half.
Roll the pastry over so the folded part is on the bottom and place on a baking sheet. Score the top of the strudel with a sharp knife.
Brush the top of the pastry with the egg and milk, then sprinkle with the remaining granulated sugar.
Bake the strudel for 25 minutes until the pastry is golden and flaky. Allow the strudel to cool for 20 minutes before serving.