Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper.
Combine the berries, lemon juice, sugar and cornstarch in a saucepan over medium heat. If desired, you can add in ½ teaspoon dried culinary lavender flowers here if you have them! Heat the mixture over medium heat. Bring it to a boil then reduce it to a simmer and cook for 5 to 6 minutes. Remove from the heat and let it cool completely. It will be thicker and more syrupy as it cools. This will take about 30 minutes of hands-off time. Once it cools, stir in the vanilla extract. You can make it the day before too. Or you can even submerge the pot in an ice bath so it cools quicker.
Place a sheet of the puff pastry on the parchment and pierce it all over with a fork. Spread the cooled fruit mixture over the sheet, leaving a 1-inch border. Brush the border with water. Place the other sheet of puff pastry carefully over top, lining it up with the bottom sheet. Use a fork to press the edges together and make them stick.
Carefully poke a few holes in the top of the pastry with a fork. This is just so it doesn’t bubble up. If it does bubble up, it’s no big deal - you can just pierce the pastry once it comes out of the oven. Brush the entire top with the egg wash.
Bake for 25 to 30 minutes. Remove the pan and let it cool slightly before glazing it. Slice and serve!