Put the apples in a bowl and mix in the lemon juice, sugar, cinnamon, nutmeg, and cornstarch and let it sit for 15 mins to release water
Transfer the filling mixture to a sauce pan and reduce the liquid over medium to low heat for around 15 mins, stirring regularly, then turn off the gas
While the filling is sitting / reducing, start the crust by putting the flour, salt, sugar, and pieces of butter in a food processor
Run the food processor until it resembles a coarse sand (5-10 seconds) then start dripping in the ice water until the mixture starts to chunk together
You'll know when it's done if you can pinch the dough with your hand and it sticks together
Preheat oven to 350°F then start making the pie crust
Cut the dough in half and roll both halves out with a rolling pin until they are big enough to cover your pie pan (around 9 inch diameter)
Put the bottom half dough into the pie pan and cut off the excess dough with a knife
Dump the sauce pan full of the filling into the pie pan and even it out
Cover the pie filling with the top dough and fold over and tuck in the excess crust
Use a fork or your fingers to pinch down the top crust to the bottom crust so you don't have air gaps
Optionally you can now brush on the egg wash to make the crust get a shiny golden brown color when it cooks
Cut a few slits in the top crust with a knife to give it a nice design and allow for air to release
Bake 45 minutes at 350°F or until the crust is brown to your liking and the apples are bubbling inside