In a saucepan on medium low heat add the strawberries, water, sugar and cornstarch and mix. Cooke 3-4 minutes and using a potato masher or fork smash the strawberries a bit.
Preheat oven to 400 degrees
Sprinkle some flour on a cutting board and lay out the puff pastry. Using a rolling pin or cup roll out the creases in the puff pastry. Cut the puff pastry into four large squares.
Add about 2 tablespoons of the strawberry filling into the middle of the squares.
Fold the corner over to form a triangle. Using a fork crimp the edges to seal.
Transfer the turnovers to a baking sheet and brush the top with the egg whites. Using a small knife make to small slit in the top.
Bake 15 minutes