Make the Filling: In a small saucepan, combine the diced strawberries, granulated sugar, cornstarch, and lemon juice. Cook over medium heat, stirring constantly, until the mixture thickens and begins to bubble. Remove from heat and let the filling cool completely.

Prepare the Puff Pastry: Unfold the thawed puff pastry sheet onto a lightly floured surface. Using a knife or a pizza cutter, cut the sheet into 4 equal squares.

Assemble the Turnovers: Spoon a generous amount of the cooled strawberry filling onto one side of each pastry square. Brush a little egg wash along the edges of the pastry.

Fold and Seal: Carefully fold the empty side of the pastry over the filling to create a triangle. Gently press the edges together with your fingers, then use the tines of a fork to crimp and seal them tightly, which helps prevent the filling from leaking out.

Bake the Turnovers: Place the assembled strawberry turnovers on a baking sheet lined with parchment paper. Brush the tops of the turnovers with the remaining egg wash. Bake at 400°F (200°C) for 15-20 minutes, or until the pastry is golden brown and puffed.

Glaze and Serve: While the turnovers cool, whisk together the powdered sugar and milk or water in a small bowl until you have a smooth, thin glaze. Drizzle the glaze over the warm strawberry turnovers and serve immediately.