Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Roll out the thawed puff pastry sheet on the prepared baking sheet.
In a bowl, gently toss the apricot slices, blueberries, granulated sugar, cornstarch, vanilla extract, lemon zest and juice until well combined.
Arrange the fruit mixture in the center of the puff pastry, leaving about 2 inches of the border around the edges.
Using kitchen scissors, cut diagonal slits into edge of pastry. Carefully fold the edges of the pastry over the fruit, creating a rustic, free-form tart.
Brush the folded edges of the pastry with the beaten egg to give it a golden shine. If desired, sprinkle crust with granulated or coarse sugar.
Bake the crostata for 20-40 minutes or until the pastry turns golden brown and the fruit filling bubbles. Check at intervals if filling overflows and starts to burn.
Once baked, let it cool slightly on the baking.
Optional - Dust the crostata with powdered sugar for an attractive finishing touch.
Optional - Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.