To make the crust

1. In a medium bowl, whisk together flour, sugar and salt. Add cold, cubed butter and, using your fingers, work the butter into the flour mixture. Quickly break the butter down into the flour mixture. Some butter pieces will be the size of oat flakes; some will be the size of peas. Create a well in the mixture and pour in the egg yolk and cold buttermilk. Use a fork to bring the dough together. Try to moisten all of the flour bits. On a lightly floured work surface, dump out the dough mixture. It will be moist and shaggy. That’s perfect. Divide the dough in two and gently knead into two cohesive disks. Wrap each disk in plastic wrap and refrigerate for 1 hour. Allowing the dough to rest in the refrigerator will help rechill the butter and distribute the moisture.
2. To make the filling, wash and slice peaches. In a medium bowl, combine peach slices and blueberries. In a small bowl, whisk together sugar, spices, flour, and cornstarch. Pour the sugar mixture over the fruit, and gently toss together with a wooden spoon. Stir in the lemon juice. Place bowl of fruit in the fridge to rest while you roll the crust out.
3. Preheat oven to 400 degrees F. Place a rack in the center of the oven, and place a baking sheet on the lower rack, just below where you’re going to place the pie. This will catch any pie drippings without making a mess of your oven.
4. Remove one of the pie dough disks from the fridge. On a lightly floured surface, roll dough out into about a 13-inch round. The dough may be a bit crumbly, so be sure to knead it well before rolling. Roll the dough a few strokes, then use your fingers to move the emerging circle around the floured surface. This ensures that the dough isn’t sticking to the work surface. The circle won’t be perfect, that’s ok. Try not to get any tears in the rolled out dough, but if you do, they can be patched together with extra dough. Gently lift the 13-inch round from the floured surface and center in the 9-inch baking dish. Place in the fridge while you roll out the top crust.
5. Roll out the top crust just as you did the bottom crust, moving the dough across the floured surface every once in a while, and creating a roughly 13-inch circle. Remove the bottom crust and fruit filling from the fridge. Gently pour the fruit filling into the pie dish. Carefully remove the top crust from the work surface and drape over the fruit in the pie dish. With a small knife, trim the crust, leaving about 3/4-inch overhang. With your fingers press the top and bottom crusts together and fold under. Use a fork or your fingers to crimp the edges of the dough. Cut five small slits in the top of the crust so the juices and steam can vent. Brush lightly with beaten egg and sprinkle with sugar.
6. Place pie in the oven and bake at 400 degrees F for 15 minutes. Reduce the oven heat to 375 and bake for 45 more minutes. Remove from the oven when crust is browned and golden, and the juices are bubbling. Remove from the oven and allow to cool for 2 hours before serving. Place covered in the fridge to store. Pie lasts up to 3 or 4 days.