In a food proccessor put the flour, sugar, and salt and process until well combined. You can also do this by using a fork to combine the ingredients, just don´t use your hands, as your body temperature will mess with it.
Now add the butter and process until the mixture is sand-like (again, you can also do it using a fork).
Then slowly add the cold water and the white vinager and mix again until a soft dough is formed.
Wrap this dough in plastic kitchen paper and leave it rest in the fridge for 1 hour.
Take the dough out of the fridge and cut it in half.
Put one half in the fridge again and roll out the other one in a floured surface.
Place the rolled out dough in a pie mold previously greased and give it a little shape using your hands. Let this sit in the fridge for 15 min.
For the apple filling:
Place the apple slices in a bowl, add the lemon juice, granulated sugar, brown sugar, cinnamon, salt and mix.
Cover up and let it sit for 30 min.
Now sift the apples, to get about 1/2 cup juice.
Pour this juice in a small pan along with the butter and cook to medium heat for 5 min. aprox. until a caramel forms.
Put the sifted apples in a bowl and add in the corn starch. Then add the caramel and mix so that every apple has caramel.
To assemble:
Preheat the oven at 400° C.
Take the pie crust out of the fridge and put the rolled oats on top of it (this will absorb the juice the apples release while in the oven, and this way the crust won´t get that humid).
Take the rest of the dough out of the fridge and roll it out (this will be the covering).
Now pour all the apples onto the pie crust with the rolled oats and then place the covering on top.
Using a knife cut any axcess dough and crimp the pie perimeters for it to look prettier.
Using the same knife, cut 5 lines about 2 inches long from the middle out, this is to allow steam to circulate out of the pie while baking. Garnish with enough egg and then srpinkle granulated sugar on top.
Wrap the pie with plastic kitchen wrap and let it sit in the fridge for 15 minutes.
Bake at 400°F for about 50 min. until the pie is golden brown.