For crust: Roll out the dough 1/8 inch thick on a lightly floured surface turning it constantly so that it does not stick; add more flour as needed (the dough might crack around the edges, most of the cracks don't matter since the edges will be trimmed after the tart is in the pan, if there are some big ones that will show when the tart is in the pan, patch them with pieces of dough before fitting the) when the dough is rolled out, flip it over the rolling pin and transfer it to a 10-inch tart pan with removable bottom; fit it into the tart pan without stretching; roll the rolling pin over the top of the tart to cut off the dough hanging over the pan's edge; refrigerate 20 minutes.
Preheat oven to 400°F Place the tart pan on shallow baking sheet; fit the tart pan with aluminum foil or parchment paper, fill with beans or pie weights, and bake in the middle of preheated oven for 15 minutes; carefully remove the foil with beans and bake for another 5 minutes.
For filling: Turn the heat down to 375F; quarter, core, and peel the apples; cut into 1/4" lengthwise slices; toss them in a bowl with the sugar and cinnamon, then arrange them in the crust in concentric circles; bake in the upper third of preheated oven for about 20 minutes, or until they start to color and are almost tender; remove from oven and let cool while preparing the custard.
For custard: Beat the egg and sugar together in a mixing bowl until mixture is thick, pale yellow, and falls back on itself forming a slowly dissolving ribbon; beat in the flour, then the cream, and finally the cognac; pour the mixture over the apples; it should come almost to the top of the pastry shell; return to oven for 10 minutes, or until cream begins to puff.
Sprinkle with icing sugar (placed in a sieve and sprinkled evenly over) and return to oven for 15 to 20 minutes more.
Tart is done when top has browned and a knife plunged into the custard comes out clean; unmold the tart; cool 10 minutes and serve.