Preheat the oven to 375 degrees F and line a baking sheet with parchment paper.

Add the strawberries, blackberries, and blueberries to a large mixing bowl with 2 tablespoons of sugar, cornstarch, and lemon zest. Stir gently. 

Unroll the pie crusts on a lightly floured cutting board and cut out 4-inch circles.

Place them on the lined cookie sheet and fill the center of each circle with berries, stopping ½ inch from the edge of the crust.

Fold the crust edge up to cover the fruit, creating pleats.

Brush the edges with the beaten egg and sprinkle with the remaining 2 tablespoons of sugar. 

Bake until golden brown, about 20 minutes. 

Store in an airtight container in the fridge for up to 3 days.