To make the pastry, put the flour, sugar and butter into a food processor or stand mixer. Mix on a low speed until mixture resembles very coarse breadcrumbs.
Add the egg yolks, cold water and vanilla, and process until a dough forms.
Take a third of the dough, shape it into a disc and wrap in plastic wrap. This will be your lattice. Form a disc with the remaining dough and wrap it in plastic wrap. This will be your crust. Refrigerate for at least an hour. In the meantime, make your filling.
To make the pastry cream, pour the milk into a medium saucepan and scald the milk – heat until bubbles appear around the edges, but it is not boiling.
While the milk is heating, mix together the sugar, flour and salt in a small bowl. In a large heatproof bowl, whisk together the egg yolks and vanilla. Whisk the flour mixture to the egg yolks slowly. It will be thick and pasty.
Once the milk is heated, slowly pour it into the egg mixture, whisking continuously. Once it is all incorporated, pour it back into the saucepan and heat over a medium heat. Whisk continuously and vigorously until the mixture thickens and comes to a boil.
Once the mixture is thickened and just beginning to bubble, whisk vigorously for 10 seconds and then turn off the heat, This cooks out the taste of flour.
Strain the mixture through a fine sieve into a heatproof bowl. Cover with plastic wrap, putting the plastic right onto the surface of the pastry cream. Refrigerate for at least 4 hours, or until cold.
To make the filling, take the stems off the strawberries and quarter them. In a large bowl, mix the strawberries together with the blueberries, sugar, tapioca flour, cornstarch and salt. Set aside.
When you are ready to bake, preheat the oven to 180 C. Lightly grease a cast iron skillet or pie dish with butter.
Remove the bigger disc of dough from the fridge, and on a lightly floured surface, or between two sheets of baking paper, roll out the dough until it is big enough to line your pie dish.
Lightly flour the dough and gently roll it up onto your rolling pin. Then unroll it into your pie dish, easing it in and pressing it into any edges. Trim the excess pastry, leaving a 2cm (1 inch) overhang.
Fill the pastry shell with the pastry cream, spreading it around to make an even layer. Then top with the berry filling, leaving the juice behind in the bowl.
Remove the smaller disc of dough from the fridge. On a lightly floured surface or between two sheets of baking paper, roll out the dough until it is big enough to cover your pie dish. Cut the dough into 1.5 – 2 cm wide strips. Arrange them in a lattice over the apples. Check out my lattice tutorial for how to do this.
Fold the excess pastry back in over the edges of the lattice and pinch to make a fluted edge.
Bake for 40-50 minutes, until the pie is golden. Allow to cool before cutting, so that the juices don’t run. Serves 8-10.