Tina Lau's Mixed Berry Pie

Pastry Dough:

2 2⁄3 cup Flourist Sifted Red Fife Wheat Flour (or another type of Red Fife Wheat Flour, or all-purpose flour)
1 tsp sea salt
1 cup and 2 tbsp cold unsalted butter, cut into 1⁄2-inch cubes
1 tbsp apple cider vinegar
1⁄2 cup ice cold water, or as needed

Combine Sifted Red Fife Wheat Flour (or another type of flour) and salt in a food processor bowl and chill in the refrigerator for 30 minutes. Mix apple cider vinegar and water and keep chilled. Chill a large metal mixing bowl and fork.
Add half of the cubed butter to the flour. Pulse 5 times and then process for 5 seconds. Add the remaining cubed butter and repeat. Butter pieces should vary in size, ranging from a pea to nickel, with each piece of butter coated in flour. Tip flour into the chilled large metal bowl.
A tablespoon at a time, drizzle 3 tablespoons of the vinegar-water mixture along the side of the bowl, allowing it to trickle towards the flour mixture. Use the chilled fork to flick the flour mixture towards the center of the bowl, rotating the bowl as you go. Then repeat. The flour mixture should start forming larger clusters.
After adding 6 tbsps. of liquid, take a handful of the flour mixture in your hands and squeeze gently. If it easily holds together without falling apart, enough liquid has been added. If it easily breaks apart, continue adding 1 tbsp of liquid at a time, checking the consistency after each addition until the just dough holds together. Add liquid sparingly.
Divide in half and form into two circular discs, wrap individually with plastic wrap. Chill for at least 1 hour preferably, overnight.

Filling:

1 lb each mixed berries - raspberries, blueberries, blackberries
2⁄3 cups (130 g) brown sugar
¼ cup (50 g) sugar
6 tbsp (45 g) cornstarch
¼ tsp salt
1 tbsp (14 g) butter, cold and cubed
1 Granny Smith apple
1 egg white
turbinado or Demerara sugar for sprinkling

Assemble and bake:

Lightly butter a pie pan.
Remove dough from the fridge. Lightly flour your rolling pin and the surface you’ll be rolling the dough out on. Roll one disc into a circle about 13 inches wide and approximately 1⁄8-1/4" thick. Gently ease the dough into the prepared pie pan, lightly pressing it into the contours of the pan and making sure there is about 1/2-inch overhang all around. If any part tears or comes away, simply press the dough back together. Place in refrigerator and repeat rolling with the second disc, placing the rolled-out dough onto a baking sheet and into the refrigerator.
Gently toss berries together in a large bowl. In a small bowl, mix both sugars together, along with the cornstarch and salt. Lightly toss the berries with the sugar-cornstarch mixture and gently tip into pie pan. Dot with 1 tbsp of cubed butter.
To create a lattice top, cut the second rolled out pie dough into strips. Starting with the longest strips in the middle of the pie, weave the strips under and over to create a lattice pattern. Work quickly as the pie dough will soften. Trim the extra dough, leaving a 1-inch overhang. Fold the top layer of dough under the bottom layer, tucking the two layers together. Decorate the edge as you wish - pressing with fork tines, fluting, crimping etc.
Place the whole pie in the refrigerator to chill for 1 hour.
To make an egg wash, lightly whip up the egg white with 15ml water with a fork. Using a pastry brush, brush the bottom of the pie dough with egg wash (this helps to protect the dough from becoming soggy during baking).
Preheat oven to 400°F. When ready to bake, generously brush the surface of the pie and the edges with egg wash and liberally sprinkle with turbinado sugar. Place pie on a baking sheet and bake for 60-90 minutes until deep golden brown. Let cool completely before slicing and serving.