Smash berries. Stir in sugar and 1/4 cup flour. Pour filling into pie crust.
Smash walnuts into small pieces. Stir in brown sugar, 1/4 cup flour, and shortening with a pastry blender. The mixture should be crumbly. Sprinkle streusel over pie.
Place pie on a baking sheet and bake in preheated oven until crust is golden and filling is bubbly, about 45 minutes.
