Preheat the oven to 200`c. Place the cubed apple, sugar and cinnamon in a saucepan with 3 tablespoons of water and cook down gently for 10 minutes, or until the apple has broken down to a puree. Put aside to cool down. Meanwhile, roll the puff pastry out on a clean work surface to a large sheet, 3mm thick. Using a bowl or plate, cut a circle about 12inches/ 30cm in diameter. Crimp the edge before turning the whole sheet over and laying it directly onto a flat baking tray. Chill in the fridge for at least 10 minutes. Remove the pastry from the fridge and spread the stewed apple compote all over the base of pastry, leaving a 1cm/½in border at the edge. Slice the apples the thickness of 3 mm and place them onto the pastry sheet, fanning them out, starting from the outside and working in. The apples should overlap each other. Use the largest slices on the outside and place the smallest slices in the middle of the tart. Once all the apples have been laid out, brush all over with the melted butter and sprinkle over the caster sugar. Brush the border with the milk and bake in the oven for about 30-35 minutes, or until golden-brown and risen around the edge.