Preheat oven to 400°F. Line a large baking sheet with parchment paper or a silicone baking mat and set aside.
Unroll both pieces of puff pastry and slice each piece into four equal sized squares, you will end up with 8 square pieces.
In a large mixing bowl, toss together the strawberries, granulated sugar, and cornstarch until all of the berries are well coated. Evenly distribute the strawberries, chocolate, and Nutella between all 8 pieces of puff pastry, making sure to keep the filling a little closer to one corner. Fold the other corner over the filling, making sure to keep the filling tucked inside, and crimp the edges with a fork to seal.
Brush the top of each turnover with the egg white and place the turnovers onto the prepared baking sheet. I usually bake these in two separate batches. Bake at 400°F for 15-18 minutes or until puffed and lightly golden brown. Remove from the oven and allow to cool for about 10 minutes.
To make the glaze:
Add the chopped chocolate and Nutella to a microwave safe bowl. Microwave in 20-30 second increments, making sure to stir well after each increment, until melted and smooth. Top each turnover with the glaze.