Wash and chop strawberries and rhubarb. Combine in a saucepan with sugar, lemon juice, and cornstarch. Cook until thick and bubbly, about 5-7 minutes. Let cool.

Preheat oven to 400°F (200°C). Roll out puff pastry on a floured countertop and cut into 4x4 inch squares.

Place filling on one side of each pastry square, fold and seal edges. Crimp with a fork.

Brush with egg wash. Bake for 20-25 minutes until golden brown.

Let cool on wire rack. Optionally glaze with powdered sugar and vanilla extract before serving.