First, prepare the pie crust. Place the cold butter, all-purpose flour, salt and sugar into a food processor. Pulse for a few minutes, until fine crumbs form. You can also use a pastry cutter or two large forks for the same process.
Gradually begin adding the water into the food processor, 1 tablespoon at a time, until dough begins to hold its shape. Keep mixing until the dough comes together into a ball. Remove the dough onto lightly floured work surface. Shape it into flat disk, wrap with plastic wrap and set it into refrigerator to chill for at least 30 minutes.
Making the filling. First, thinly slice the peaches or nectarines. Next, prepare the crumble: in a large bowl, combine the flour, cornstarch, butter, sugar, nutmeg and cinnamon. Scrap the seeds from the vanilla bean and add to crumble (can also use 1 teaspoon vanilla). Use a pastry cutter to cut the butter into the dry ingredients until fine crumbs from.
Preheat the oven to 375F. Remove the dough from the fridge and cut it in half. Roll out the first half of the dough on a lightly floured work surface, making it approximately 2-inch larger than your pie pan. Gently lift the dough off the work surface and line your pie pan, leaving the extra dough hanging off the edges.
Roll out the second half of the prepared pie crust. Using a rule or a long, sharp knife, cut into the dough into equal strips and arrange in lattice pattern over pie. Watch my video recipe to see how it's done. Crimp the edges and discard leftover dough. Fill the pie alternating with the sliced peaches and the crumble until the pie is filled.
For the egg wash: Beat together the egg and water in a small ramekin. Using a pastry brush, brush the egg wash over dough. Sprinkle the large sugar crystals over the egg wash before it dries to ensure the sugar adheres to the surface.
Arrange a thin strip of foil around the edges (or use pie crust protector) of the pie to prevent the edges from burning. Bake the peach pie for 45 minutes or until bottom of pie is golden and ready. Remove the from the oven and let it cool for at least 1 hour before cutting; for best results, cool for several hours or overnight. Serve the peach pie warm with vanilla bean ice cream.