Preheat an oven. In a large bowl, combine shredded mozzarella, ricotta cheese, cottage cheese, feta cheese, chopped spinach, chopped Castelvetrano olives, Kosher salt, and large eggs. Stir until very well mixed.
Place each sheet of puff pastry on a piece of parchment paper, then roll out slightly so it’s just barely smaller than the sheet pan. Carefully transfer one piece of parchment to the sheet pan.
Use a spatula to spread the cheese and spinach mixture evenly on top of the puff pastry. Carefully take the other rolled out sheet of puff pastry and, using the parchment paper to lift it, invert it onto the filling.
Cut diagonal lines in one direction across the entire length of the pan. You do not need to cut all the way through to the bottom. Rotate the pan and cut diagonal lines in the other direction.
Place remaining egg and heavy cream in a small bowl, then whisk with a fork. You can sprinkle the entire sheet pan with topping or alternate diamonds with various toppings. Transfer to a preheated oven and bake.
Then cut along the diagonal lines to loosen each piece. Serve warm or at room temperature!