To serve 12 people, you will need to make two pies.
On a lightly floured surface, roll out the flaky pie pastry into two 12-inch rounds 1/8 inch thick. Fit 1 round into a 9-inch glass pie plate. Transfer the other round to a baking sheet lined with wax paper. Refrigerate until ready to use.
Preheat the oven to 375°. In a bowl, toss the berries with the sugar, flour, lemon juice and cinnamon. Pour the filling into the lined pie plate. Lightly brush the rim of the bottom crust with water and drape the top crust over the filling. Press the dough edges together and trim the overhang to 1/2 inch, fold the overhang under and crimp decoratively.
Using a sharp knife, make a 1-inch slash in the center of the top crust. Bake the pie for 1 hour, or until the filling is bubbling and the crust is golden. Transfer to a wire rack to cool before serving.