Prepare the pie crust and refrigerate for 30 minutes or up to 3 days. (See recipe below) Line a large baking sheet with foil and set aside.
When ready to bake, preheat oven to 375°F. Whisk together the sugar, cornstarch and lemon zest. Place the sliced nectarines and blueberries in a large mixing bowl. Add the sugar/cornstarch mixture, lemon juice and grated apple to the fruit. Using a large rubber spatula, gently fold to combine. Set aside while rolling out the crust.
Roll out one of the pastry discs in a circle that's big enough to allow a 1-inch overhang. Trim the excess dough with kitchen shears. Fill the pastry crust with the nectarine mixture spreading into an even layer. Roll out the remaining pastry and cut into shapes using a cookie cutter. Place in a random pattern over the filling as desired.
In a small bowl, whisk together the egg yolk and milk. Brush the crust "stars" and around the top edge of the crust (see video). Sprinkle with sugar.
Place the pie on the foil lined baking sheet and bake for 75-90 minutes or until the crust is browned and the filling is bubbling. Check the pie after about 45-60 minutes and if browning too quickly, tent loosely with foil.
For best results, allow the pie to cool to room temperature before serving. Serve with ice cream if desired. Refrigerate leftovers and enjoy!