Remove your pastry dough from the freezer or fridge to get to room temperature. You just want to make sure it's not frozen because if you try to roll it out it will break.
Preheat your oven to 375 degrees.
Lay out your puff pastry on a flat surface that has been sprinkled with flour to avoid sticking. Cut your pastry sheet into 3x4 rectangles and set onto a baking sheet.
Put about 1 tbsp of pesto on each pastry (add more if your heart desires!), leaving about 1/2 inch empty on every side of the pastry.
Put 1/4c shredded mozzarella on top of your pesto on each pastry.
Slice your heirloom tomatoes and lay 2 slices on top of your mozzarella. Use the remaining of your mozzarella cheese to sprinkle on top of your tomatoes.
in a small bowl, whisk your egg and 1 tsp of water. Using a pastry brush, brush the edges of your pastries and then sprinkle with flakey sea salt.
Bake for 20-25 minutes until edges are golden brown.
Remove from oven and shred fresh parmesan on top, drizzle balsamic glaze, and top with fresh basil. Enjoy!