Prepare the filling by washing and chopping strawberries and rhubarb, then combining in a saucepan with sugar, lemon juice, and cornstarch. Cook until thickened.

Preheat the oven to 400°F (200°C). Thaw and roll out the puff pastry, cutting into squares.

Spoon filling onto pastry squares, fold, and seal edges. Repeat with remaining squares.

Place turnovers on a baking sheet, brush with egg wash, and bake for 20-25 minutes until golden brown.

Let cool slightly, dust with powdered sugar if desired, and serve warm.