Preheat your oven to 425°F (220°C).

In a large mixing bowl, gently toss together the chopped apple, chopped rhubarb, mixed berries, granulated sugar, cornstarch, and lemon juice. Stir until the fruits are well-coated, and set aside.

Roll out half of your prepared pie crust on a lightly floured surface until about 1/8 inch thick. Fit the crust into a 9-inch pie plate and trim excess dough.

Pour the berry mixture into the prepared crust, spreading it evenly.

Moisten the edge of the bottom crust with a little water, roll out the other half of the pie crust, and cover the filled pie, crimping the edges together.

Make several slits in the top crust to allow steam to escape, then brush with egg wash.

Place the assembled pie on a baking sheet and bake for 15 minutes at 425°F (220°C).

Reduce the oven temperature to 375°F (190°C) and continue baking for another 35-45 minutes.

If the crust begins to brown too quickly, tent it with foil.

Once golden and bubbly, remove from the oven and let it cool completely on a wire rack.