Roll out pre-made pie crust according to direction on the package and place it in a pie pan. Crimp edges with a fork.
In a medium saucepan, whisk together the sugar, cornstarch, water, and salt. Fold in the blueberries. Bring to a boil over medium heat. Once boiling begins, cook for 2-3 minutes or until the mixture thickens. Allow mixture to cool slightly.
Preheat oven to 400°.
While the blueberry mixture is cooling, assemble the crumb topping. Combine flour, sugar, cinnamon, ginger, and salt in a medium bowl. Add butter and use your fingers to mix it into the dry ingredients until pea-size clumps form. Set aside.
Fold raspberries, strawberries, blackberries and lemon juice into the blueberry mixture. Pour mixture into the pie crust.
Top the fruit with the crumb mixture — scattering it evenly.
Bake pie for 10 minutes at 400°. After 10 minutes, reduce heat to 350° and bake 45-50 minutes or until crust is golden brown and filling is bubbly.