Place the flour, sugar, salt and butter in a bowl.
Use your fingers to pinch the butter with the rest of the ingredients until there are no large pieces of butter left and it looks like wet sand.
Add the egg and water at the center and mix with a spatula until it doesn't help anymore and then mix with your hands. Don't over knead it because it will turn tough and hard to roll out after.
Wrap the dough and let it rest in the fridge for at least 30min. It you leave it longer than that and it goes hard then leave it outside of the fridge for a while until it becomes softer. If it's hard at the edges and soft inside you want to give it a mix with your hands on an unfloured surface.
Dust a surface with flour and roll it out until it's 3mm thick. You don't want to roll it thinner than that because the juice could then break it in the oven. Make sure that the dough isn't sticking to the surface with each roll and add more flour if needed. Try to make a circular shape as you go. Alternatively you can roll the dough between two baking paper sheets so the dough won't break.
Place the dough on a baking tray lined with baking paper or a silpat. If you stretched the dough between baking paper than you just need to peel the top one off and transfer the dough with the bottom paper onto a tray.
At this point you can leave the dough as is with its rustic ends or use a large dish as a mould and cut around it with a knife. It's really important that the dough is on the baking tray before placing on the apples because once they're on you won't be able to transfer it over.