1-Preheat your oven: Preheating your oven to 400°F (200°C) and lining a baking sheet with parchment paper to avoid any sticking.

2-Prepare the fruit filling: Combining the chopped apples, rhubarb, sugar, cornstarch, and lemon juice in a bowl; mix everything thoroughly so the flavors blend well.

3-Roll out the puff pastry sheets: Roll out the puff pastry sheets on a lightly floured surface and cut them into even squares aim for about 6×6 inches each. Spoon the fruit mixture onto the center of each square, leaving space around the edges for sealing. Fold the squares over the filling to form triangles, then press the edges with a fork to seal them securely, which helps prevent leaks during baking. To finish, brush the tops with the beaten egg for a golden shine and cut a few small slits on top to let steam escape, ensuring your turnovers bake up perfectly flaky.

4-Chill and bake: Place the assembled turnovers on your prepared baking sheet and chill them in the refrigerator for 20 to 40 minutes, depending on how cold your filling is this step is crucial for that ideal flaky texture. Bake them for 20 to 25 minutes until they’re golden brown and the filling is bubbling. After baking, let them cool slightly before serving; for an extra treat, drizzle with a simple icing made from powdered sugar and lemon juice. Remember to prepare the filling ahead if you can, as chilling it overnight makes the process even smoother.