In a large bowl, whisk together the 2 1/2 cups all-purpose flour, 1 tsp sugar, and 1/2 tsp salt. Add the cold, diced 1 cup unsalted butter and cut it into the flour using a pastry cutter or your fingertips until the mixture resembles coarse crumbs.
2 1/2 cups all-purpose flour, 1 tsp sugar, 1/2 tsp salt, 1 cup unsalted butter
Gradually add the 6-8 tbsp ice water, 1 tablespoon at a time, mixing just until the dough comes together. Divide into two discs, wrap in plastic, and refrigerate for at least 30 minutes.
6-8 tbsp ice water
In a medium bowl, combine the 1/2 cup strawberries, 1/2 cup raspberries, 1/2 cup blackberries, 1/2 cup blueberries, 1/3 cup granulated sugar, 1 tbsp cornstarch, 1 tsp lemon juice, 1/2 tsp vanilla extract, and 1 tsp salt. Stir gently to coat the berries evenly. Let sit while you prepare the dough.
1/2 cup strawberries, 1/2 cup raspberries, 1/2 cup blackberries, 1/2 cup blueberries, 1/3 cup granulated sugar, 1 tbsp cornstarch, 1 tsp lemon juice, 1/2 tsp vanilla extract, 1 tsp salt
On a floured surface, roll out one disc of dough to about 1/8-inch thickness. Cut into 3x6 inch rectangles. Repeat with the second disc of dough, re-rolling scraps as needed. From half of the rectangles, cut thin strips to create a lattice topping.
Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
Place a heaping tablespoon of the berry filling in the center of each solid dough circle, leaving a 1/4-inch border. Lay the lattice strips over the filling, weaving them into your desired pattern. Trim the edges and press to seal. Crimp with a fork or your fingers.
Whisk the 1 egg and 1 tbsp water together. Brush over the lattice and edges of the pies. Sprinkle with coarse sugar, if desired. Bake for 20-25 minutes or until the crust is golden brown and the filling is bubbling. Let cool slightly before serving.
1 egg, 1 tbsp water