To make filling:
Combine the 1 lb (455 grams) cranberries, 1 lb (455 grams) blueberries, 1 ½ cup (300 grams) granulated sugar, 3 tablespoons cornstarch, 2 cinnamon sticks, ¼ teaspoon freshly grated nutmeg, juice and zest from 1 lemon in a large saucepan.
Cook over medium heat until mixture comes to a boil, about 10 minutes, stirring occasionally as berries soften and release juices. Continue to cook for 2 minutes until thickened, stirring constantly.
Set aside to cool, removing cinnamon sticks. If filling it too hot it will melt bottom crust, but it's ok if it's still just a bit warm. (Filling can be made up to 1 week ahead. Store in the fridge.)
To bake pie:
Preheat oven to 400°F.
Roll out one half of the double crust recipe into a 15-inch circle. Fit into a 9-inch pie dish and trim edges to 1-inch overhang. Roll out second half of crust and cut into strips for a lattice, if desired. Alternately, leave pie crust whole for a standard top crust.
Pour cooled filling into the prepared pie dish and spread into an even layer.
Criss-cross the strips of pie crust to form a lattice (see Notes). Alternately, place whole crust on top and cut small vents for steam to escape. Brush with the egg wash made from 1 egg + 1 tablespoon water and sprinkle generously with sugar (if using)
Bake until filling is bubbly and crust is golden, about 1 hour. If crust browns too quickly, cover with foil so it doesn't burn.
Let pie cool before serving so filling has time to set. Serve warm or at room temperature, with whipped cream or ice cream, if desired.
Store leftovers in the fridge for up to 5 days. To reheat, cover with foil and bake at 350°F for 30 minutes until warmed through.
Calories: 261kcal, Carbohydrates: 54g, Protein: 2g, Fat: 5g, Saturated Fat: 2g, Cholesterol: 16mg, Sodium: 78mg, Potassium: 111mg, Fiber: 4g, Sugar: 37g, Vitamin A: 75IU, Vitamin C: 16mg, Calcium: 22mg, Iron: 1mg