Sieve the flour with the baking powder and put it on a working surface, make a well in the middle and add the sugar, the salt and the grated peel of one lemon. Beat the egg in a bowl and pour it in the well. Mix the egg with the sugar using your fingers to make a soft dough. Add the softened butter and mix it to the egg and sugar mixture.
When it is totally mixed, rub all the ingredients with your fingertips and make crumbles, then start kneading until you have a nice and smooth ball of dough. Roll it out roughly between two foils of parchment paper and place it in the refrigerator for at least 2 hours.
Making the custard
Heat the milk on medium with the lemon peel: heat until the steaming point. Whisk the eggs with the sugar and flour. Remove the lemon peel from the hot milk and pour it over the egg mixture and bring back to the heat, stirring constantly with a wooden spoon. As soon as the custard is thick and veils the spoon, remove from the heat, cover with cling film and let cool down.
Assembling the tart
Heat the oven to 180°C. Grease a 24 cm round baking tin. Take the shortcrust pastry out of the fridge, unwrap it and roll it out. To help roll the dough, keep the dough on top of the parchment paper and dust with the flour. This can help rolling the dough and can also help when transferring the dough to your pan.
Prick the bottom of the pastry shell with a fork. Spread the cake with the apricot jam, then cover it with a thick layer of cusatard and sprinkle with cinnamon.
Peel the apples and remove the core, then slice them into 5 mm thick slices. Cover the custard with the apple slices, arranging them as you prefer, but making sure that each slice is slightly overlapping the previous one, so as to completely cover the cake. Dust generously with sugar.
Bake the cake for 50 minutes or until the cake is golden and the apples are cooked and lightly caramelized. Remove the cake from the oven and let it cool completely before serving!