1. Make the Pastry Crust: Place the flour, butter and powdered sugar in a food processor and process until the mixture resembles wet sand, or coarse breadcrumbs. With the motor running, add the egg yolks and ice water, and process until a dough just comes together. Turn the dough out onto a lightly floured surface and bring together using your hands. Divide the dough in half and flatten into discs. Wrap tightly in plastic wrap and refrigerate for 1 hour.
2. Preheat the oven to 325 degrees F.
3. Make the Filling: In a large bowl, gently mix together the sliced nectarines, sugar, cornstarch, honey and vanilla extract; set aside.
4. Assemble the Pie: If you have refrigerated the dough for longer than 1 hour, it may need to sit at room temperature for 10 to 15 minutes to be workable. When ready, roll out 1 pice of the dough between 2 sheets of parchment paper into an 17x12-inch rectangle. Gently transfer the dough to an ungreased 15x10-inch jelly roll pan. Sprinkle the surface of the dough with the almond meal (or oat flour) and top with the nectarine mixture, gently spreading it into an even layer.
5. Roll out the remaining piece of dough as you did above and place it over the top of the pie. Press the edges of the dough together and use the tines of a fork to seal. Refrigerate for at least 1 hour.
6. Using a sharp knife, cut a cross in the middle of the pastry. Brush the beaten egg evenly over the surface of the dough and sprinkle with the sugar. Bake until golden brown, about 55 to 60 minutes. Allow to cool for at least 20 minutes before serving.