Place the flour, salt, sugar, and butter in a food processor and pulse until the dough becomes crumbly with pea-sized pieces of dough. With the processor running, slowly add the water until the dough forms a ball.
Remove the dough and divide into two pieces, one slightly larger than the other. Form each dough ball into a disk and wrap tightly with plastic wrap. Refrigerate for 6 hours or overnight.
Preheat the oven to 400° F and position one oven rack on the bottom and one in the center.
In a large bowl, toss the sugar, cornstarch, cinnamon, vanilla, nutmeg, salt, and peaches together to combine. Set aside.
On a lightly floured surface, roll out one dough disk to approximately 11 x 14 inches. Gently fit the dough into an ungreased 9 x 13 x 1 sheet pan. Do not trim excess overhang.
Pour the peach mixture into the sheet pan and spread evenly.
Roll out the remaining dough disk to approximately 11 x 14 inches and cut into 1-inch strips. Place the dough strips on top of the pie and weave to create a lattice top. Pinch the edges of the two crusts together to seal and tuck under any overhang. Trim the excess dough with a knife.
Brush the top of the dough with egg wash and sprinkle with sugar.
Bake for 35-40 minutes or until the crust is golden brown and the filling is bubbling.
Transfer to a cooling rack and cool completely before cutting.
