Sift together the flour, confectioners' sugar, and salt into a large mixing bowl.

Use a pastry cutter or two knives to work the butter into the flour until it resembles coarse meal.

Stir in the egg with a fork until the dough just begins to hold together.

Press the dough into a rough ball, then transfer to a lightly floured surface.

Knead the dough a few times, gather it into a ball, then flatten slightly to make a disk.

Wrap the disk in plastic and refrigerate for 30 minutes.

Preheat the oven to 375°F (190°C).

Peel, quarter, and core the apples.

Melt the butter in a 10" cast-iron skillet over medium heat, then remove from heat and sprinkle evenly with sugar.

Tightly pack the apples around the inside edge of the skillet, standing upright on their sides, nestled against one another.

Arrange the remaining apples in a similar position in the center of the skillet. (The apples will shrink as they cook.)

Return to high heat and cook until the butter and sugar caramelize to a rich brown, about 15 minutes.

Remove from heat, then carefully turn the apples over to the uncooked side with a fork, taking care not to burn your fingers.

Return to heat and cook for another 5–8 minutes.

Remove from heat.

Roll out the dough on a lightly floured surface into a 12" circle.

Drape the pastry over the apples and skillet, then tuck the overhanging dough between the apples and the inside wall of the skillet.

Bake in the oven until the pastry is golden, about 20–30 minutes.

Allow the tart to cool for 15 minutes, then loosen the edges with a knife.

Place a platter on top of the skillet and invert quickly and carefully.

Serve warm.