In a large bowl, combine the flour and salt.

Cut in the butter with a pastry blender (or rub in with your fingers) until the mixture resembles coarse crumbs.

Working quickly, stir in the sour cream, then bring together with your hands just until a dough has formed.

Wrap well with cling film and refrigerate for at least 30 minutes, or up to 1 day.

Just before assembling the pie, combine the rhubarb, sugar, cornstarch and orange zest in a large bowl and set aside.

Divide the dough in half and, on a floured surface, roll out one portion into a ⅛-inch (3 mm) thick circle.

Line a 9-inch (23 cm) pie dish with the dough and brush the edges lightly with egg wash.

Add the rhubarb filling and dot with the butter.

Roll out the second portion of dough and drape over the filling.

Press the edges together, trim to a 1-inch (2 ½ cm) overhang, then tuck under and crimp.

Brush with egg wash and sprinkle with sugar, then cut slits in the top for venting.

Refrigerate the pie for 30 minutes while you preheat the oven to 400°F (200°C). Place a foil lined baking tray on the middle rack in the oven to pre-heat.

Bake the pie on the pre-heated tray for 55-60 minutes, or until the juices are bubbling and the crust is golden brown. (If the crust is browning too quickly, loosely tent with foil).

Cool for at least 4 hours before slicing to allow the filling to set.

Enjoy at room temperature with a dollop of whipped cream. Store leftovers loosely covered at room temperature overnight or refrigerate for up to 2 days.