Dust your work surface with flour, roll out pastry to 3mm thickness, divide into two rectangles then cut each one into three.
Place the pastry rectangles on a baking tray lined with baking parchment and roll the sides of each one up slightly to form borders. Prick the bottom of each tartlet with a fork. Peel the apples, removing cores and pips, then grate into medium strips and mix with half of the caster sugar.
Spread a layer of compote on each tartlet, then share the grated apple out on top. Sprinkle with remaining sugar and add a few small chunks of butter to each one.
Bake in the middle of the oven for 20 minutes, then reduce the heat to 350°F (180°C) and bake for a further 20 minutes. Leave in the oven for a further 5 minutes, with the door half open.
Leave tartlets to cool completely on a wire grill, once you have taken them out of the oven.