Make the crust: cut cold butter into flour/sugar/salt to pea-sized pieces; add ice water until dough holds; divide into 2 discs, wrap, refrigerate 1 hour.
Prep the apples: toss slices with lemon juice, sugar, flour, cinnamon, nutmeg, salt; rest 20 minutes so juices draw out.
Line the dish: roll larger disc to 12 inches; ease into 9-inch pie dish without stretching; refrigerate.
Fill and dot: pile rested apple mixture in, pour in the juices, scatter butter dots over the top.
Weave the lattice: cut second disc into ten 1-inch strips; weave lattice; trim, fold edge, crimp; refrigerate 15 min.
Glaze and bake: preheat oven to 425 F with tray inside; brush lattice with egg wash, sprinkle coarse sugar; bake 20 min, reduce to 375 F, bake 35 min more until deep golden and bubbling.
Cool before slicing: cool on a wire rack at least 1 hour (2 better) so the filling sets.