Great question — answering as the UphillTech team:
The unusual part of the Ferrandi method is NOT the apples; it is the attempt to control the puff pastry separately. Most tarte Tatin recipes put the pastry directly over the cooked apples and bake the whole assembly together. For example, Food Network says to “Gently place on top of the cooked apples” before the final bake. That makes the book's separate pastry case make more sense: it gives Ferrandi much tighter control over the pastry's shape and degree of puffing, while the apples can be cooked until properly concentrated without simultaneously worrying about the pastry. There is precedent for separating the components; Chef's Pencil, for example, specifies cutting the puff pastry “Using...
Full answer by user266652: https://cooking.stackexchange.com/a/137300 (CC BY-SA 4.0).
Need it done for your business? UphillTech.