A golden, buttery double-crust pie brimming with blueberries, raspberries, and blackberries set into a jammy, sliceable filling. Pre-cooking the fruit ensures bright, concentrated berry flavor and a clean slice every time.
Make a double-crust pie dough and chill for at least 2 hours.
Preheat oven to 425°F (218°C). Place a baking sheet on the rack below the pie to catch drips.
In a saucepan, combine most of the blueberries, raspberries, and blackberries with granulated sugar and cinnamon. Cook over medium heat until berries release juices.
Spoon out a few tablespoons of hot juices into a bowl and whisk in cornstarch to form a smooth slurry.
Return the slurry to the saucepan and cook, stirring, until the mixture thickens to a jam-like consistency.
Remove from heat and stir in the remaining fresh berries, lemon zest, lemon juice, and vanilla. Let cool slightly.
Roll out the bottom crust, fit into a pie dish, and add the berry filling.
Add the top crust (lattice or design of choice). Crimp or flute edges to seal.
Brush top and edges with egg wash; optionally sprinkle with coarse sugar.
Bake at 425°F (218°C) for 20 minutes. Reduce to 375°F (190°C), shield the crust edges, and bake until the filling bubbles across the surface and crust is golden, about 50 minutes more (internal temp around 200°F/93°C).
Cool completely at room temperature for at least 5 hours before slicing. Serve plain or with vanilla ice cream.