Begin by washing, trimming, and chopping fresh strawberries and rhubarb. In a medium saucepan, combine the fruit with granulated sugar, lemon juice, and cornstarch. Stir continuously over medium heat until thickened, about 5-7 minutes. Remove from heat and cool.

Thaw the puff pastry according to package instructions. Roll out on a floured surface until smooth. Cut into 4-5 inch squares.

Spoon a generous amount of filling onto one half of each pastry square. Fold over and press edges to seal. Crimp edges with a fork.

Preheat the oven to 400°F (200°C). Arrange turnovers on a lined baking sheet. Brush tops with beaten egg. Bake for 20-25 minutes, until golden brown.

Cool on a wire rack. Dust with powdered sugar if desired, and serve warm or at room temperature.