Make the crust:

1. Cut the cold butter into 1/2-inch thick cubes. It should be fresh from the fridge.
2. Combine the flour, salt, and butter in a food processor. Pulse several times until the mixture is crumbly, but still has large chunks of butter.
3. Add the egg, vinegar, and 2 teaspoons of ice-cold water. Pulse until the dough begins to hold together. If needed, add additional water 1 teaspoon at a time.
4. Turn the dough out onto the counter. Press it into a ball, then flatten it into a disc about 5 inches wide. Wrap it in plastic wrap and refrigerate it for a minimum of 60 minutes.
5. Roll the dough onto a lightly floured surface, rolling from the center to the edges and rotating occasionally. Roll it out so it’s about 11” in diameter, leaving you at least 1 inch on each side of the pie plate.
6. Place the dough into a 9-inch pie plate and fold under the excess edges, then crimp the edges. Refrigerate the dough while you prepare your pie filling up to 60 minutes is best after rolling.
7. Preheat to oven to 375°

Make the filling:

1. Peel and slice the apples about 1/8” thick and toss with the lemon juice.
2. In a separate bowl, combine the dry ingredients, and then toss the apples thoroughly in the dry ingredients. Finally, drizzle the maple syrup over top and add to the prepared crust.

Assemble the topping and bake:

1. Dice the butter into small pieces and combine with the flour, sugar, and cinnamon. Use a pastry cutter or two forks and press the butter into the dry ingredients until it resembles a crumbly mixture and the butter is broken down into smaller pieces.
2. Next, add the rolled oats and cut them into the flour mixture. Once it’s roughly combined, use your hands to start pressing the mixture into larger pieces.
3. Sprinkle over prepared pie filling once it’s been added to the crust. 
4. Bake at 375°F for 50-60 minutes. Cool before serving. Once cooled, keep covered.