Special equipment: 6 wide-mouth (3 1/2-inch diameter) mason jar lids and a 4 1/4-inch round cutter, optional
Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper. Prepare 6 wide-mouth (3 1/2-inch diameter) mason jar lids so that the rubber seals on the inner part of the lids face down, then spray the insides of the lids lightly with cooking spray.
Toss together the peaches, blueberries, sugar, cornstarch, nutmeg and a pinch salt in a medium bowl. Drizzle with the lemon juice and toss again. Set aside at room temperature while you roll the crusts.
Roll out 1 crust on a lightly floured surface. Cut out 6 bottom crusts with a 4 1/4-inch round cutter (or just cut slightly larger than the circumference of the lids). Fit the crusts into the prepared lids and press up the sides so the crusts slightly overhang the edges. Roll out the second crust and cut out 6 top crusts. Cut 3 small slits in each top crust for steam vents.
Divide the filling among the bottom crusts and lightly brush the edges with the beaten egg. Dot with the butter, top with the vented crusts and crimp to seal. Brush the tops lightly with beaten egg and sprinkle with demerara sugar. Bake on the prepared baking sheet until the crusts are golden brown and the filling is bubbly, about 30 minutes. Let cool at least 20 minutes before serving.
To serve, press the bottom of each lid up and pop the pie out, leaving the ring and lid behind.