To make a pie using frozen berries, combine the berries with sugar, lemon juice, cornstarch and sugar. Place the berry mixture into a pie pan containing a pie crust. Add in a dab of butter, place the top crust on the pie, and bake at 350 degrees Fahrenheit.
Preheat the oven to 350 F. Unroll a pre-made pie crust into a pie plate. In a large bowl, mix together 8 cups of frozen berries, 1/2 cup of sugar, 1/4 cup of cornstarch and 1 tablespoon of fresh lemon juice. Pour the berry mixture into the pie crust, and dot with 2 tablespoons of unsalted butter. Cover with the top pie crust, and crimp the edges. Cut slits in the top crust to vent the steam as the pie bakes.
Mix together the yolk of 1 egg and 1 tablespoon of heavy cream. Brush the egg mixture across the top of the pie. Place the pie on a cookie or baking sheet to catch any spills, and bake for 30 minutes. Increase the oven temperature to 375 degrees F, and bake for an additional 20 minutes or until the crust is a golden brown color. Allow the pie to cool or serve warm.