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Put the Bramley apple, sugar and cinnamon in a small pan with 1 tbsp water. Cover with a lid and simmer over a low heat for 10-15 mins until the apple breaks down and turns into a compote.
Stir in the butter and the Gala apple and cook for a further 5 mins, uncovered, until the Gala apple is just soft but retaining its shape, and any excess water has evaporated to make a thick compote around the Gala chunks. Remove from the heat and leave to cool.
Heat the oven to 220C/200C fan/gas 7. Unravel the puff pastry sheet out on a work surface, and cut into 4 even rectangles. Divide the apple filling to one side of each of the puff pastry rectangles, leaving a 1cm border. Brush the border with milk and fold over the empty side of pastry over the filling and crimp together. Brush with more milk and sprinkle over the sugar. Make a small steam hole in the middle of the turnovers.
Transfer to a parchment-lined baking sheet and bake for 20-25 mins until golden and puffed up. Leave to cool for 10 mins on a wire rack (the filling will be molten when it first comes out the oven) before serving warm with a scoop of vanilla ice cream, or serve at room temperature. Keep for 2 days in an airtight container.