In a cast iron skillet, combine 100 grams of the sugar and 2 tablespoons water. Place the skillet over medium heat and stir until the sugar dissolves. When the sugar starts crystallising, whisk the mixture vigorously until the it turns into a fairly dark caramel.
Make sure the bottom of the skillet is coated in caramel and set it aside to cool.
Preheat the oven to 400°F/200°C.
For the filling, arrange the apple slices in concentric circles in the caramel, sprinkle with the remaining sugar, and dot with the butter.
On a lightly floured surface, roll out the pastry to about a 5mm (¼inch) thickness and lay it over the apples, tucking the pastry into the skillet so that the fruit is completely covered.
Bake for 30 minutes, then immediately flip onto a plate so that the apples are on top. Serve warm or room temperature, with cream or ice cream.