Melt butter over medium-low heat. Add the apples to the pan with 3 tbsp of granulated sugar plus corn starch, and cinnamon.
Sauté the apples for around 5 minutes, until they're coated with sauce and reduced. Turn off the heat, stir in the raisins, then leave the apples to cool completely.
Roll out the puff pastry into a rectangular shape. Pile the apple mixture at the center of the pastry, leaving an inch of pastry at the sides and the top apple-free.
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