Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Unfold the thawed puff pastry sheets onto a lightly floured surface. Cut each sheet into 9 equal squares (18 squares total).
Transfer the squares to the prepared baking sheet. Using a sharp knife, gently score a border about 1/2 inch from the edge of each square, being careful not to cut all the way through.
Place a small pinch of shredded mozzarella inside the border of each square to act as a base.
Layer the sliced beef, zucchini rounds, tomato slices, and mushrooms on top of the cheese.
Sprinkle the dried herbs, salt, and black pepper over the toppings.
Brush the edges of the pastry with the beaten egg wash to ensure a golden shine.
Bake for 15-18 minutes, or until the pastry is puffed and golden brown.
Remove from the oven and let cool for 5 minutes before serving warm.