Preheat the oven to 200°C, fan 180°C, gas 6. Line a baking tray with baking paper.
Roll out the pastry then use a saucer about 18cm in diameter as a guide for you to cut out two circles of pastry, then re-roll the trimmings and cut out two more circles.
Score a smaller circle about 1cm in from the edge of the pastry circles to help the outside to rise and the inside to stay flat.
Peel the apples, core and finely slice. Spread a little custard over the middle of each pastry circle, saving some for serving, and then top with the apple slices in a circle – cram them in tightly. Sprinkle the sugar over the top.
Bake for 15 minutes – check the tarts halfway through and, if the apples are turning brown, lower the oven to 180°C, fan 160°C, gas 4.
Heat the remaining custard in a pan and serve with the tarts.