Instructions

for the filling

 Combine peaches and reserved juice, sugar, and salt in medium saucepan. Using potato masher, crush one-third of peaches. Cook over medium heat, stirring occasionally, until sugar is dissolved, about 5 minutes.
 Stir lemon juice and cornstarch in small bowl until well combined. Add mixture to saucepan and cook, stirring constantly, until mixture comes to simmer and juice thickens, 30 to 60 seconds. Transfer to bowl and refrigerate until fully cooled, about 1½ hours. Stir in lemon zest.

for the dough

Place flour, granulated sugar, and 1 teaspoon salt in 1-gallon, heavy-duty zipper-lock bag. Seal and shake well to combine. Add butter to bag and shake to coat with flour mixture. Seal bag, pressing out as much air as possible. Set rolling pin over lowest portion of bag and, using rocking motion, flatten butter beneath pin into large flakes. Working in sections, move pin up bag and flatten remaining butter. Shake bag to mix. Roll over bag with pin, shaking bag occasionally to mix, until flour becomes very pale yellow and almost all of butter is incorporated. Transfer mixture to large bowl (use rubber spatula or bench scraper to scrape any remaining butter and flour mixture from bag). Add ice water and toss with rubber spatula until just combined (mixture will be tacky). Transfer dough to floured counter. With your floured hands, press dough into rough 8-inch square.
 Roll dough into 15 by 10-inch rectangle with short side parallel to edge of counter, flouring counter and dough as needed. Starting at top of dough, fold into thirds like business letter, using bench scraper or metal spatula to release dough from counter. Turn dough 90 degrees and repeat rolling and folding. Divide dough in half crosswise. Wrap each half tightly in plastic wrap and refrigerate for at least 1 hour or up to 2 days.
 Adjust oven racks to upper-middle and lower-middle positions and heat oven to 400 degrees. In small bowl, beat egg and remaining pinch salt until well combined. Line 2 rimmed baking sheets with parchment paper.
 On lightly floured counter, roll 1 piece of dough into 17 by 9-inch rectangle with short side parallel to edge of counter. Roll dough loosely around rolling pin, turn 90 degrees, and arrange on counter so long side is parallel to edge of counter. If dough has contracted, roll again briefly to achieve 17 by 9-inch dimensions. Using pastry brush, apply 1-inch-wide band of egg wash horizontally across center of dough. Apply 1-inch-wide strip of egg wash to edges of lower half of dough. Apply 1-inch-wide bands of egg wash vertically to divide bottom half of dough into 4 equal squares.
 Place 2 tablespoons filling into each square, being careful to keep filling clear of egg wash. Using spoon, shape filling into rough squares. Using pizza cutter or sharp knife, cut dough at center points between filling to create 4 pieces. Leaving pieces in place but working with 1 piece at a time, gently fold dough over, aligning top edge with bottom edge. Use your fingers to gently press dough layers together, working out as much air as possible. Trim ¼ inch of dough from ragged and folded sides of each pie. Arrange pies on prepared sheet, leaving at least ¾ inch between them.
 Repeat steps 6 and 7 with remaining dough and filling. Cut 1-inch vent on top of each pie. Transfer pies to refrigerator to chill for 15 minutes. (Pies can be frozen on baking sheets until solid, at least 4 hours, and then transferred to airtight container and frozen for up to 6 weeks. Transfer to parchment paper–lined rimmed baking sheets and thaw in refrigerator for 2 hours before proceeding with step 9.) 
 Brush tops of pies with egg wash and sprinkle with demerara sugar, if using. Bake until dark golden brown, 20 to 25 minutes, switching and rotating sheets halfway through baking. Let cool for 20 minutes; serve warm.