Apple Blackberry Slab Pie

Name: Apple Blackberry Slab Pie
Ingredients: 4 cups (560g) flour; 2 tablespoons granulated sugar; 1 1/2 teaspoons salt; 1 teaspoon baking powder (preferably aluminum-free); 1 1/2 cups (12 ounces, 340g) unsalted butter (cubed and chilled); 2/3 cup (160ml) ice water (plus more if necessary); 12 cups (3 pounds, 1,3kg) peeled, sliced or cubed apples; 2-3 cups (8-12 ounces, 225-340g) blackberries; 2/3 cup (170g) granulated sugar; 2 tablespoons cornstarch; 4 teaspoons freshly squeezed lemon juice; 2 tablespoons Calvados, Cognac, or brandy (or 2 teaspoons vanilla extract); 1 egg; 1 teaspoon milk; 3-4 tablespoons granulated natural sugar (see headnote) (or granulated sugar)