1 Make the pie dough and chill: Cut the cold butter into cubes and set aside for 5 minutes to take the chill off. In a large mixing bowl, whisk together the flour, salt, and sugar. Add the cubes of butter and work them into the flour with your fingers.
Pinch and rub the cubes of butter together until the mixture resembles coarse meal with some chunks of butter. Add the sour cream and use a fork to mix it into the flour-butter mixture until it forms big curds of dough.
Turn the dough out onto a counter and use your hands to gather it into a large ball. Cut it in half with a knife and pat each half into a thick rectangle. Sprinkle each rectangle with a little flour, wrap in plastic wrap, and refrigerate for at least an hour or up to a day ahead (or freeze and then thaw overnight in the fridge). (See detailed instructions with photos for making the dough in our Sour Cream Pie Crust recipe.)
2 Roll out the dough: Roll out 1 of the rectangles of dough into a thin rectangle roughly 11 x 15 inches (slightly larger than your pan). Gently transfer the crust to a quarter-sheet pan or jelly roll pan, and fit it into the corners. The crust should hang over the sides by about an inch. Do not trim.
Transfer the pan to the fridge and refrigerate for 15 minutes.
3 Prepare the apple filling: Peel and core all of the apples. Cut them into 1/2 inch chunks. In a large mixing bowl, combine the cut apples, sugar, cornstarch, cinnamon, and a pinch of salt.
Remove the bottom crust from the fridge and spread the apple filling evenly over the surface of the crust.
4 Roll out the top crust: Remove the remaining rectangle of dough from the fridge and roll it into a large rectangle roughly 10×13 inches (about the same size as your pan). At this point, you have two choices:
Traditional Top Crust: Transfer the rolled-out dough over the apples. Roll and crimp the edges. Cut six vents in the top; three on top and three on the bottom.
Lattice Top Crust: Cut the rolled-out dough into strips and weave or braid the strips into a decorative lattice top crust. Roll and crimp the edges.
5 Chill the pie and preheat the oven: Transfer the pie to the fridge and chill for 30 minutes while the oven heats. Heat the oven to 375F.
6 Brush the top of the pie with egg wash: In a small bowl, whisk together the egg and milk. Brush the top of the pie with the egg wash and sprinkle with sanding sugar.
7 Bake the pie: Place the pie on top of a larger sheet pan or baking sheet to catch any drips. Bake for 50 to 60 minutes, or until the top is golden and the center of the pie is bubbling.
8 Cool completely: Cool the pie completely before serving, about 45 minutes.
The fully baked pie can also be covered in foil, stored on the counter, and served the next day. Warm briefly in the oven if desired before serving. Leftovers will also keep, wrapped in foil, for about 2 days at room temperature or about 5 days refrigerated.