For the crust:

1. In a medium-sized bowl, mix together the flour and salt. Dice the butter into small pieces and crumble it with the flour until lumps are the size of small peas. Add sugar and stir well. Pour in the iced cold water, little at a time, mixing with a fork. Finish by hand and shape into a ball. Divide in two equal parts and wrap each of them into cling film. Place in the refrigerator and chill for about 30 minutes.
2. Take the first dough and roll out into a until the dough is 3 inches (7 cm) longer than the pan you are using, and around 1/4″ (5 mm) in thickness. Carefully fold the dough, lay it across the pie pan, and gently unfold the crust, using your hands to fit the dough down into the pan. Burst any air bubbles with a fork.
3. For the lattice strips: On a lightly floured work surface, roll out the second pie dough into a 12-inch (30 cm) circle.
4. Using a pizza cutter or pastry roller, slice the dough into large strips (the width of the strips is totally up to, and you can make 2 additional braids with the strips to display around the pie dish if you wish). Lay the strips on a prepared baking sheet and refrigerate until needed.

For the filling:

1. In a large mixing bowl, combine blackberries and apple slices together with sugar, cornstarch, vanilla extract, lemon juice and zest. Mix carefully so that all fruits are well-coated and let sit for 20 minutes.
2. Preheat the oven to 425°F (220°C).
3. Using a rubber spatula, scrape the blackberry-apple filling into the pie crust shell. Top with diced butter.
4. Prepare a lattice pie crust following this step-by-step tutorial, and add 2 braids around the pie dish (optional but so worth it when you see the final result!). In order to seal the braids and the edge of the pie, gently pat with water underneath the braids.

For brushing:

1.  Beat together the egg and water in a small bowl until well combined. Brush the edges of the crust with the egg wash and sprinkle with sugar.
2. Bake for 15 minutes at 425°F (220°C), then reduce the oven temperature to 350°F (180°C) and continue baking for 50 minutes, or until the crust is golden and the filling is bubbling. Let cool at room temperature for at least 30 minutes before slicing.