Roll a store bought pie crust on to a piece of parchment paper. Set to the side.
Preheat the oven to 400 degrees Fahrenheit.
In a large bowl, combine the mixed berries, granulated sugar, cornstarch, lemon juice. Mix gently until the berries are evenly coated.
Pour the filling into the center of the dough, leaving a 2- to 3-inch border. Gently fold in the edges of the crust all along the circle.
Using a whisk, mix the egg until scrambled. Brush the crust with egg and sprinkle with Turbinado sugar.
Bake until the crust is golden and the filling just begins to bubble, about 25 minutes. Let cool 20 minutes and serve warm or at room temperature.