Instructions

 Roll 1 disk of dough into 12-inch circle on floured counter. Loosely roll dough around rolling pin and gently unroll it onto 9-inch pie plate, letting excess dough hang over edge. Ease dough into plate by gently lifting edge of dough with your hand while pressing into plate bottom with your other hand. Leave any dough that overhangs plate in place. Wrap dough-lined plate loosely in plastic wrap and refrigerate until firm, about 30 minutes.
 Roll other piece of dough into 13 by 10½-inch rectangle on floured counter, then transfer to parchment paper–lined rimmed baking sheet; cover loosely with plastic and refrigerate until firm, about 30 minutes.
 Using pizza wheel, fluted pastry wheel, or paring knife, trim ¼ inch dough from long sides of rectangle, then cut lengthwise into eight 1¼-inch-wide strips. Cover loosely with plastic and refrigerate until firm, about 30 minutes. Adjust oven rack to middle position and heat oven to 400 degrees.
 Toss peaches, ½ cup sugar, lemon zest and juice, and salt in bowl and let sit for at least 30 minutes or up to 1 hour. Combine pectin, cinnamon, nutmeg, and remaining 2 tablespoons sugar in small bowl; set aside. Measure out 1 cup peach pieces and mash with fork to coarse paste. Drain remaining peach pieces through colander set in bowl, reserving ½ cup peach juice. Return peach pieces to now-empty bowl and toss with cornstarch.
 Whisk reserved peach juice and pectin mixture together in 12-inch skillet. Cook over medium heat, stirring occasionally, until thickened slightly and pectin is dissolved (liquid should become less cloudy), 3 to 5 minutes. Transfer peach-pectin mixture and peach paste to bowl with peach pieces and stir to combine. Spread peach mixture into dough-lined plate.
 Remove dough strips from refrigerator; if too stiff to be workable, let sit at room temperature until softened slightly but still very cold. Space 4 strips evenly across top of pie, parallel to counter edge. Fold back first and third strips almost completely. Lay 1 strip across pie, perpendicular to second and fourth strips, keeping it snug to folded edges of dough strips, then unfold first and third strips over top. Fold back second and fourth strips and add second perpendicular strip, keeping it snug to folded edge. Unfold second and fourth strips over top. Repeat weaving remaining strips evenly across pie, alternating between folding back first and third strips and second and fourth strips to create lattice pattern. Shift strips as needed so they are evenly spaced over top of pie. (If dough becomes too soft to work with, refrigerate pie and dough strips until firm.)
 Trim overhang to ½ inch beyond lip of plate. Pinch edges of bottom crust and lattice strips together firmly to seal. Tuck overhang under itself; folded edge should be flush with edge of plate. Crimp dough evenly around edge of plate. (If dough is very soft, refrigerate for 10 minutes before baking.) Brush surface with egg wash.
 Place pie on aluminum foil–lined rimmed baking sheet and bake until crust is light golden, 20 to 25 minutes. Reduce oven temperature to 350 degrees, rotate sheet, and continue to bake until juices are bubbling and crust is deep golden brown, 30 to 50 minutes longer. Let pie cool on wire rack until filling has set, about 4 hours. Serve.