Preheat the oven to 375°F and line a rimmed baking sheet with parchment paper.
In a medium mixing bowl, use a baking spatula to fold together the ricotta, lemon zest, parmesan, and chopped fresh basil. Use a microplane grater to grate the garlic into the filling. Season the filling with salt and pepper, folding it in to distribute it evenly. Taste the filling and adjust the seasoning as needed.
If the squash blossoms have long stems, trim these off at the base of the flower. Using a knife, gently make an incision through one side of the flower, then use your hands to gently open the flower so it is flat. Lastly, the stamen (pollen stem) inside the flower needs to be removed. Use your fingers to gently pinch out the stamen.
Unroll the sheet of butter puff pastry on the lined baking sheet. Use a fork to prick holes all over the sheet of pastry. Scoop the ricotta filling into the center of the pastry and spread it out into an even layer, leaving about an inch of crust on all four sides.
Place the butterflied squash blossoms over the filling so the outsides of the flowers are facing up. Whether you need four or six squash blossoms to completely cover the filling will depend on the size of the blossoms. Gently fold the pastry over itself onto the squash blossoms and filling, repeating on all four sides.
In a small bowl, beat together 1 large egg and 1 tablespoon of water. Use a pastry brush or basting brush to apply the egg wash to the tart's crust.
Transfer the tart to the preheated oven and bake for 35 minutes, rotating the pan halfway through the baking time.
Remove the tart from the oven and allow it to cool on the pan for 5 minutes before slicing and serving. Serve the tart warm.