Make the crust: Prepare a half batch of my gluten-free pie crust or you can make the full batch and freeze half for later, which is what I always do.
Preheat your oven to 425°F (220°C).
Prepare the pie crust: Use a rolling pin to press down on the pie crust disk a few times to soften it. Dust your rolling pin and a piece of parchment paper with gluten-free flour. Roll out the bottom crust into a large circle onto the piece of parchment paper, until it is about ¼ inch thick and 12 inches in diameter for a 9-inch pie dish.
Place the pie dish upside down on the center of the rolled-out dough. Using the parchment paper for support, carefully flip the dough and pie dish over so the dough lines the inside of the dish. You want to have about 1.5 inches of overhang on all sides. Take the overhang and roll it under itself to create a thick edge.
Using your fingers, crimp or flute the edges of the dough.
Place the crust in the freezer to chill while you prepare the filling and topping.
Make the peach filling: Peel the peaches (see notes below for instructions to easily peel peaches). Then, cut them into 1.5-2 inch chunks. Place the peach chunks in a large bowl.
Add the granulated sugar, brown sugar, cornstarch, cinnamon, nutmeg, lemon juice, and vanilla extract to the peaches. Toss everything together until the peaches are evenly coated. Set aside.
Make the streusel topping: In a separate bowl, mix together the gluten-free flour, granulated sugar, brown sugar, cinnamon, and salt.
Add the melted (slightly cooled) butter and stir with a fork until the mixture forms a crumbly texture. Set aside.
Assemble the pie: Remove the pie crust from the freezer. Pour the peach pie filling into the pie crust, spreading it out evenly.
Sprinkle the streusel topping evenly over the peaches.
Egg wash: In a small bowl, whisk together the egg and water. Lightly brush the edges of the pie crust with the egg wash to give it a golden color during baking.
Bake the pie: Place the pie on a baking sheet to catch any drips, and bake for 15 minutes at 425°F (220°C)
After 15 minutes, lower the oven temperature to 375°F (190°C) and continue baking for an additional 35 to 55 minutes, or until the crust is golden and the filling is bubbling. If the edges of the crust begin to brown too quickly, cover them with foil or a pie shield.
Remove the pie from the oven and let it cool on a wire rack for at least 2 hours to allow the filling to set.
Slice and serve the peach crumble pie as is, or with a scoop of vanilla ice cream.