In a large bowl, whisk together the pastry flour, sugar, baking powder, and salt. Drizzle in the oil — or cut in the margarine — and, using a fork, whisk and press together until the mixture resembles a coarse meal. Add the water 1 tablespoon at a time, continuing to lightly stir with the fork, until the dough just comes together. Press into a ball, wrap in plastic wrap, and let rest in the fridge at least 20 minutes.

Remove the crust from the fridge and divide it into two pieces. On a well-floured board, using a well-floured rolling pin, roll out one-half of the crust in a circle and fit it into a 9″ pie pan (leave a bit of crust to hang over the edges — but just a little bit). Pour in the filling. Roll out the remaining dough into a large rectangle, and, using a serrated knife, gently cut equally thick strips of dough. Weave the strips across the top of the pie to make a lattice crust. (Alternatively, roll another circle of pie crust into a circle and fit atop the filling. If doing this, cut several air vents in the top of the pie.)