Make sure the frozen blackberries are completely thawed. Pour ½ cup of the liquid into a measuring cup and set aside.
6 cups blackberries, ½ cup reserved blackberry juice
Place place the berries, sugar, cornstarch, lemon juice, salt, and ginger into a large pan. Stir constantly and cook over low heat until the sugar and cornstarch dissolve. If the blackberries don't release extra liquid, pour in the ½ cup reserved juice. If they do release extra liquid, discard the reserved juice.
1 ¼ cups granulated sugar, ⅓ cup cornstarch, 1 tablespoon freshly squeezed lemon juice, ½ teaspoon salt, ¼ teaspoon ground ginger
Adjust the heat to medium. Continue to cook until the mixture thickens and begins to bubble. Remove from heat and allow to cool for 15 to 20 minutes.
Optional: For a super silky pie filling, stir in 2 tablespoons of butter.
2 tablespoons unsalted butter
While the filling cools, use a pastry brush to apply the egg wash to the bottom, sides, and edges of the unbaked pie crust.
1 large egg, 1 pie crust
Scoop the warm filling into the crust and top it with the crumble topping. Pinch the crumble mixture into small pieces and place them over the filling.
Bake at 400°F for 50 to 60 minutes until the topping turns a deep golden brown and the filling is bubbly.
Allow the pie to cool for 1 hour at room temperature before transfering to the refrigerator to cool completely (about 2 to 3 hours).