Mix the cut fruit, sugar, and lemon juice together in a large bowl, and set aside for 20-30 minutes, or until a few tablespoons of juice has been released.
2-2 ½ cups strawberries, cored and cut into small pieces, 2-2 ½ cups blackberries, cut into pieces if they are large, 2-2 ½ cups blueberries, ½ cup sugar or brown sugar, or a combination, 1 Tbl lemon juice
Pour some of the juice into a small bowl and whisk it together with the cornstarch until smooth. Set aside.
⅓ cup cornstarch
In a saucepan, add all of the fruit mixture and begin to cook on medium heat. When the mixture begins to simmer, stir in the cornstarch mixture. Allow to heat up to a boil, and cook for 3-5 minutes or until the mixture thickens. Stir frequently to avoid burning. Then remove from the heat, cover, and allow to cool to room temperature. If the filling is too hot, it will melt the butter in your top crust and you'll have a gummy top crust rather than a flaky, crunchy one.
Once everything is cool, preheat your oven to 425℉ and place a sheet pan on the rack below where your pie pan will go. This will catch any accidental butter drips from the crust. You can mix up your egg wash now as well, by whisking together your egg and water in a small bowl.
1 egg, 1 Tbl water
Roll out your top crust portion of the pie dough. It will be cold from the fridge, but it should not be a struggle to roll out. If you find you're fighting to get it to move, allow it to sit on the counter for 10-15 minutes and then try again. If you are making a lattice or any other decorative top, cut out the pieces you will need now. It's ok to put them back in the fridge once they're cut if they're getting warm.
Pour the cooled filling into your par-baked pie crust. Then add your top crust in whatever design you've chosen. Be sure to cut steam release holes if you've chosen a full solid top crust. Brush on the egg wash, and sprinkle the top with turbinado sugar if you'd like.
2 Tbl turbinado sugar (optional)
Bake for 15-20 minutes at 425℉, and then reduce the heat to 375℉ and continue to bake for another 15-20 minutes. When it's done, the filling should be bubbling and the crust should be deep golden.
When fully baked, remove from the oven and cool completely before cutting. Otherwise you'll have a soupy mess. Then enjoy!