In a food processor, sift the flour, salt and powdered sugar. Add cold cubes of butter. Pulse until the mixture is like coarse sand, with a few bigger chunks of butter.
Add 3 tablespoons of ice-cold water and pulse a few times, until the dough starts clumping together, not more than that. If it doesn't start coming together, then add another tablespoon of ice-cold water.
When it's done, it should form crumbly clumps of dough - that's fine because you'll combine it together in the next step.
Transfer the dough onto a sheet of plastic wrap, loosely wrap it and then gently press it with your hands or a rolling pin to flatten it to a smooth disc.
Leave it in the fridge for at least an hour or up to four days.
to make the streusel topping
Sift and combine the flour, salt, and sugar into a small bowl. Gradually add the melted butter while stirring with a fork, until the mixture starts clumping together. Add the rolled oats, stir to combine, and set aside until needed.
to make the blueberry apple pie filling
Start with peeling and slicing the apples to approx. 3-mm (⅛-inch) thickness. Place the sliced apples to a large bowl and add the lemon zest, juice and blueberries.
In a small bowl, combine together the sugar, cornstarch, cinnamon and cardamom with a fork until every granule is evenly coated.
Add the sugar mixture to the fruit and gently stir everything with a spoon until completley incorporated.
to assemble the pie
Roll the chilled pie dough on a lightly floured surface to approx. 4-5mm (¼-inch) thickness. Turn it every now and then so it doesn't stick and it rolls evenly.
Transfer the dough to a 23-cm (9-inch) pie plate and gently press it with your knuckles to fit the pan smoothly.
Trim the overhanging excess dough using scissors, but leave approx. 2.5-cm (1-inch). Tuck the overhanging dough under itself to create a thicker edge. Chill the pie crust for 15-20 minutes in the fridge. Remove from the fridge and crimp the edges as desired.
Freeze the pie crust for at least 15 minutes until completely firm before filling with fruit.
Preheat the oven to 220°C (425°F) or 200°C (390°F) if you're using a fan-assisted oven.
Add the prepared fruit filling to the chilled pastry using a large spoon. Make sure to use all the fruit and tuck it in densely. Pour over all the juices from the bowl.
Sprinkle the top with prepared streusel. Transfer the pie plate on a baking tray lined with parchment paper.
Bake on a lower oven rack for 20 minutes, then reduce the temperature to 190°C (375°F) and bake for additional 50 minutes or so. The pie crust needs to be a deep golden brown color with bubbles from the fruit filling peeking through the crumble.
When it's done, remove the pie from the oven and let it cool to room temperature before slicing and serving.
You can use frozen blueberries instead of fresh ones.
To make sure the dough doesn't shrink while baking, freeze the crimped pie dough for at least 15 minutes before filling it with fruit and streusel.