Roll out the chilled pastry and line a 9-inch pie dish with it. Prick the pastry all over with a fork.
☞ TESTER TIP: You'll probably need to let the crust rest at room temperature for at least 10 minutes or so before rolling to warm ever so slightly so it becomes easier to manage.
Cut a piece of parchment paper into a circle about twice the size of your pie dish, crumple it, uncrumple it, and place it in the pastry-lined dish with a generous handful of pie weights or uncooked beans or rice. Bake the pastry for 10 minutes.
Remove the parchment and baking beans or rice and bake for 5 minutes more. Remove the pastry from the oven but leave the oven turned on.
Make the crumble topping
In a bowl, combine the flour, salt, cinnamon, and the sugar. Add the butter and mix it with your fingertips until a crumbly mixture forms. Mix the chopped walnuts into this crumbly mixture.
Make the bluebarb filling
In a large bowl, stir together the fresh blueberries, rhubarb, and dried blueberries (if using) along with the sugar and 3 tablespoons of the crumble mixture. (Mixing some of the crumble mixture into the filling helps absorb the rhubarb's plentiful juices and imparts a touch of buttery gooeyness to the fruit.)
Pour the filling into the pre-baked pie crust and smooth it into an even layer. Place it on a rimmed baking sheet lined with aluminum foil and bake for 20 minutes.
Remove the pie from the oven and sprinkle the fruit filling with the remaining crumble mixture. Return the pie to the oven and bake until the filling is bubbling and the crumble topping is golden brown, 20 to 30 minutes.
Wait just a few minutes before slicing and serving so the filling isn't scalding hot. If desired, top with softly whipped cream or vanilla ice cream.