Preheat the oven to 400°F (200°C).
Prepare the filling by mixing the diced strawberries, cream cheese, sugar, vanilla extract, lemon juice, and cornstarch in a bowl.
Roll out the puff pastry on a lightly floured surface to smooth out wrinkles and folds.
Cut the pastry into squares, about 4x4 inches each.
Spoon a generous amount of strawberry filling into the center of each square.
Fold the pastry over to create a triangle and press the edges to seal.
Brush the tops with the beaten egg for a golden finish.
Place the turnovers on a baking sheet lined with parchment paper and bake for 15-20 minutes, or until golden brown.
Let the turnovers cool slightly before serving.
