Position rack in the center of the oven and preheat oven to 175 degrees (or lowest setting for your oven.) Line a jelly roll pan with parchment paper or aluminum foil (to facilitate cleanup) and butter an 8 by 8-inch baking dish (oven-proof glass or ceramic is ideal); set aside Peel the apples, cut them in half from stem to blossom end, and remove the core with a melon baller. Working with the apples cut side down, cut each half crosswise into very thin 1/16 inch thick slices, keep the slices in place to retain the form of the apple half. Working with one-half apple, at a time, press the apple in your hand and fan the slices, lay the slices in the pan. Continue fanning and arranging the apple halves until you've covered the bottom of the pan. Fill gaps with apple slices. You are basically trying to get an even layer in the pan the arrangement isn't so important. Brush the layer generously with butter, sprinkle evenly with a thin layer of sugar, and toss on a few strands of orange zest. Continue making layers until you have used all the apples, at which point the ingredients may mound above the top of the pan.
Double wrap the pan with plastic film, stretching the plastic around the pan, top and bottom, to make certain that it is well sealed. Prick the plastic on the top in 6 to 8 places with a knife and weight the top with it couple of ovenproof plates or bowls, taking care not to cover all of the air holes you've created. Nesting two souffle dishes on top of the pan is a perfect arrangement. (it's OK that a portion of the surface won't be weighted.) Place the weighted pan on the lined jellyroll pan and bake for 10 hours. (Don't be concerned the oven temperature is so low there's no danger of the plastic wrap melting or burning.) Remove the apples from the oven to a cooling rack and, keeping the plastic wrap and weights in place, cool to room temperature.
Chill the apples, still wrapped and weighted, for at least l0 hours before using them. when you are ready to use the apples, unmold or spoon them from the pan.
The apples can be kept wrapped airtight in the refrigerator for about 5 days.
Adapted from "Dessserts by Pierre Herme" by Dorie Greenspan