Make the crust with a food processor: Place flour, salt, sugar, and butter in a food processor and pulse 4-5 times. Pea-sized butter flakes should still be visible. Add one tablespoon water at the time and pulse 1-2 times after every tablespoon. Stop adding water when the dough starts to clump.
If you don't have a food processor, then make the crust with a pastry cutter or two forks instead: In this case mix together flour, salt, and sugar in a large mixing bowl. Then add butter and cut it into the mixture until it resembles coarse meal with pea-sized butter flakes. Then add water little by little until the texture starts to clump.
Divide the crust in half and form 1- 1.5 inch thick discs without kneading. Cover tightly with plastic wrap and refrigerate at least 2 hours or up to 2 days.
Preheat oven to 375°F (190°C).
In a large bowl stir apple, sugar, flour, and lemon juice to combine. Also, add vanilla and cinnamon if using. Let stand 15 minutes.
Meanwhile, on a lightly floured surface, roll out the first dough disc into a 12-inch circle. Transfer to a 9-inch pie dish. The edges of the crust should hang out of the baking form. Spoon the filling into the crust without any excess liquid. Discard any excess liquid.
Roll out second dough disc and place on top of the filling. Cut slits on top that the steam can escape, or cut the second dough disc into 1-2 inch strips and arrange in a grid on top of your filling by laying over and under one another carefully. Decorate pie crust to your preference. Seal and flute the edges.
In a small bowl stir together egg and water and brush on top of the crust and sprinkle with sugar. Bake for 45 minutes. Cover the last 10 minutes with parchment paper to prevent it from browning too much if necessary. Let cool to room temperature and chill for at least 4 hours in the fridge. Store leftovers in an airtight container in the refrigerator up to 3 days or freeze for up to 3 months.