Roll one layer of dough out into a roughly 12-inch circle that is about ¼-inch thick. Carefully press the dough into your pie dish/tart tin & store in the fridge. Roll the second layer out into a rectangle roughly 12-inches in size. Place on a sheet pan & leave to chill in the fridge while you prepare the filling.
In a large bowl, gently mix together the blueberries, apples, caster sugar, cardamom, lemon juice & the cornstarch. When combined, pour the filling into the prepared pie dish & return to the fridge.
Remove the chilled rectangle from the fridge & place onto a lightly floured work surface. Use a ruler to cut long even strips into the dough. I cut my strips at different widths to vary the braid sizes. Once you have your dough sliced up, plait it into at least three different braids (I did four different styles – YouTube always helps!).
Remove the pie from the fridge & carefully transfer the braids to the top of the filling. (I had some leftover strips of dough that I didn’t plait & used these to fill the gaps between the different braids.) When all of the braids are placed on the top of the pie, crimp the edges of the dough whatever way you’d like.
Preheat the oven to 200C. While the oven heats up, slide the whole pie into the freezer for at least 15-minutes, & think of it as a necessary step to pie success.
When ready to bake, brush the top of the pie with the beaten egg yolk and sprinkle with some demerara sugar before baking for 40-50 minutes, or until the juices are bubbling & the crust is a deep golden brown.
Leave the pie to cool & serve warm with some lightly whipped cream.