2) Roll the pastry out on a floured surface to a 25cm circle. Transfer the pastry to a pie tin and press it down into the bottom and sides. Fold the edges under and flute or crimp them with a fork.
For the filling:
1) Preheat the oven to 190C/Gas 5. In a mixing bowl cut together the flour, sugar, brown sugar, shortening and butter until the crumble resembles coarse breadcrumbs; set aside.
2) In another mixing bowl whisk together the egg, treacle, hot water and bicarbonate of soda. Pour the egg mixture into the pastry shell. Sprinkle the crumble mixture on top.
3) Bake the pie for 30 to 35 minutes. The centre may still quiver a bit when jiggled; it will firm up a bit more as it cools. Serve cold.