For the crust, prepare the dough, shaping half of it into a 9- or 10-inch crust as directed but leaving the edges overhanging by 1". Chill until needed. Position a rack in the lower third of the oven, place a baking stone on the rack if you have one, and preheat to 425ºF.
For the filling, peel the apples, cut them off the core, and slice them a scant 1⁄4-inch (6-mm) thick. You should have 6 cups (900 g) total. Place the apple slices in a large bowl and gently toss with the brown sugar, lemon zest and juice, cornstarch, cinnamon, nutmeg, and salt, then gently toss in the quinces. Let sit while you prepare the crumble, 10–20 minutes, tossing a few times. Pour the fruit and their juices into the unbaked crust, packing the fruit into the crust.
Roll the second half of the dough into a 12-14” round about 3/8” thick. Cut the dough into 8 strips of the same width. Place 4 strips over the fruit, spaced evenly apart. Now weave the lattice: fold two of the strips back halfway, and lay a strip over the flat strips, crosswise. Unfold those two strips, then fold back the other strips, and lay another strip across the flat strips. Repeat with the remaining two strips. (If that makes no sense, watch this video!)
Trim the edges of the strips to a ½” overhang. Fold the bottom crust overhang over itself and the edges of the lattice strips, and use your fingers to flute the dough. Chill the pie in the refrigerator or freezer until firm, 10-20 minutes, then brush the top with the milk or cream and sprinkle with the coarse sugar.
Place the pie on the lined baking sheet and place in the oven on the baking stone. Bake at 425ºF for 15 minutes, then decrease the oven temperature to 375ºF and continue baking until the crust is golden and the fruit is bubbling furiously, 40–60 more minutes.
Let the pie cool completely to set the fruit, at least 2 hours and up to 8 hours, then cut into wedges and serve at room temperature, with scoops of ice cream if you like. The pie is best the day of baking and will keep at room temperature for up to 1 day or refrigerated for up to 3 days.
Notes
Give yourself a few hours to make the dough as it need time to chill between rollings. Don't skip the "roll, fold, roll" step, as this is what will make the gluten-free dough pliable enough to weave into a lattice.The dough can be made up to 2 days ahead of time and refrigerated (or up to 1 month and frozen), and same goes for the poached quinces.If weaving a lattice gives you the heebie jeebies even after watching this video, you can simply cut the top round of dough into 2-inch rounds and lay them on top of the fruit, overlapping a bit and leaving some windows, and proceed with the recipe.I made this in a 10-inch pie pan, but a 9-inch pan will work; you'll just have an extra-tall pie.Nutritional values are based on one of eight servings.