Preheat oven to 350°F (180°C) and line a 9-inch (22 cm) pie pan with parchment paper.
In a large bowl, whisk together the flour, salt, baking powder, and sugar. Cut in the butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Add the egg and mix until a smooth dough forms. Divide the dough in half. Roll out one half and press it into the prepared pie pan.
In a separate bowl, combine the grated apple, ground walnuts, and cinnamon. Spread the filling evenly over the crust.
Roll out the remaining dough and place it over the filling. Crimp the edges to seal. Cut slits in the top of the crust to allow steam to escape.
Bake for 35-40 minutes, or until the crust is golden brown. Let cool in the pan for 10 minutes before slicing.
Dust with powdered sugar before serving. Serve with vanilla custard or whipped cream, if desired.
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