For the filling:
16 oz. frozen wild blueberries (do not thaw)
12 oz. fresh or frozen cranberries (do not thaw)
1¼ cups sugar
3 tbsp. cornstarch
2 cinnamon sticks
1 tbsp. freshly squeezed lemon juice
½ tsp. finely grated lemon zest
2 disks pie dough for a double crust pie
Heavy cream, for brushing
Grated nutmeg, for sprinkling
To make the filling, combine the frozen blueberries, cranberries, sugar, cornstarch, cinnamon sticks, lemon juice and lemon zest in a medium-large saucepan. Cook over medium-high heat, stirring to blend, until the mixture thickens and begins to boil, about 12-14 minutes. Once the mixture reaches a boil, continue boiling for 2 minutes more, stirring constantly. Transfer the mixture to a bowl or other container and let cool completely before proceeding.
When you are ready to bake the pie, preheat the oven to 400˚ F. Roll out one disk of pie crust on a well-floured surface into a 12-inch round. Line a 9-inch pie plate with the crust. Remove the cinnamon sticks from the filling mixture, then spread the filling into the bottom pie crust. Roll out the remaining disk of pie dough and cut into thin strips. Use the strips to form a lattice over the top of the filling. Cut off the excess crust and pinch the edges together, using your fingers to create a fluted pattern. Lightly brush the top crust of the pie with heavy cream. Sprinkle grated nutmeg over the crust.
Place the assembled pie on the upper rack of the oven (top-middle position). Place a rimmed baking sheet on the lower oven rack beneath the pie. Bake, rotating halfway through baking, until the crust is golden and the filling is bubbling, about 60-70 minutes. If the crust seems to be browning too quickly, loosely tent with foil to prevent over-browning. Let cool on a wire rack to room temperature.