Preheat oven to 350 degrees F. Grease a 9 x 13-in baking pan. Set aside.
Combine filling ingredients in a large bowl and set aside.
Whisk flour, baking powder, and salt in a bowl and set aside.
Beat sugar and egg with electric mixer until light and pale. Mix in oil, milk, and vanilla.
Add dry ingredients to the egg mixture and mix until a soft dough forms. If the dough is sticky, continue kneading until it comes together.
Divide the dough in half and press it into prepared pan, making sure to extend the dough slightly up the sides of pan. Pour apple mixture, along with any juices, over dough. Spread to even out.
Roll out remaining dough to fit pan, and carefully lay the dough over the filling. Tuck the dough into the corners and using a sharp knife, make 3 slits in the center of the pie.
Make the egg wash by beating egg with 1 tbsp water and lightly brush the top of the pie crust.
Bake 45-55 minutes, or until golden brown.
Serve warm or at room temperature.