Preheat your oven to 420 degrees
Start by thawing your storebought puff pastry dough according to the package instructions. Take care to handle the delicate sheets gently to prevent tearing.
Prepare your filling by thawing your frozen spinach. Then, place the thawed spinach inside a tea towel or cheese cloth and twist the towel to remove as much excess liquid from the spinach as possible.
Place the spinach in a large bowl and add to it the egg, fresh dill, fresh parsley, onion, garlic, salt and pepper. Mix well to combine.
Dump the mixture into a bowl and fold in the feta and mozzarella cheese.
On a lightly floured, flat surface, roll out a puff pastry sheet. Cut the puff pastry sheet in half so you are left with two rectangles.
Place about ¼ cup - ½ cup of the spinach mixture in the center the puff pastry rectangle. Be sure to leave ½ an inch of pastry dough around the perimeter of the spinach mixture so you can seal the dough.
Brush the edges of the puff pastry rectangle with egg, and fold over the other side to enclose the filling. Press the edges firmly with a fork to seal.
Use a sharp knife to cut a slit at the top of the pastry, and place it on a parchment lined baking sheet, spacing 2 inches apart.
Repeat with the remaining rectangles.
Brush the top of the pastries with egg wash and sprinkle with sesame seeds
Bake for 20-23 minutes in the preheated oven, or until golden brown. Cool on racks.