In a bowl, combine the sliced peaches, brown sugar, flour, lemon juice, vanilla, and salt. Stir to coat the peaches and set aside.
In another bowl, beat together the mascarpone, powdered sugar, and lemon zest until smooth and creamy.
Roll pie crust out on a lightly floured surface or between two sheets of parchment. It should be approximately 14 inches in diameter. Fold or drape onto a rolling pin and transfer to a sheet of parchment paper.
Dollop the mascarpone in a middle and use a spoon or offset spatula to spread into an even layer, leaving about 2 inches of empty pie crust all around.
Drain excess liquid from peaches and arrange slices on top of the mascarpone. Fold over the pie crust toward the center, tucking in any overlaps to create a packet. Lift the parchment and transfer to a rimmed baking sheet.
Whisk together egg and 1 tablespoon water, then brush all over the pie crust. Sprinkle crust with sparkling sugar or coarse sugar, if desired.
Bake for 30-40 minutes, until crust is golden brown. Let cool 15 minutes before serving to allow juices to settle.
Notes
If using frozen peaches, make sure they are fully thawed and drained of excess liquid.