Prepare the fruit filling: In a medium saucepan, combine the frozen berries, sugar, lemon juice, and cornstarch. Cook over medium heat, stirring frequently, until the mixture thickens and the berries break down. Let cool completely.

Prepare the crust: In a small bowl, combine the graham cracker crumbs and melted butter. Press the mixture into the bottom of a 9x13 inch baking dish.

Assemble: Pour the cooled fruit filling over the graham cracker crust.

Freeze: Freeze for 4-6 hours, or overnight.

Serve: Before serving, let the cake thaw for about 30 minutes. Top with whipped cream and serve immediately.

Enjoy your delicious frozen fruit cake!

Tip: For a more intense flavor, let the frozen berries thaw slightly before adding them to the saucepan.