Preheat oven: Set oven to 400°F (200°C) and line a baking sheet with parchment paper for easy cleanup.

Cook filling: Heat olive oil in a skillet over medium heat. Add mushrooms and a pinch of salt; cook 6–8 minutes until deeply browned and liquid evaporates. Stir in garlic for the last minute, then add spinach and cook until wilted. Remove from heat and season with pepper.

Mix filling: In a bowl, combine cream cheese, half of the mozzarella (or Gruyère), and the Parmesan until smooth. Fold in the mushroom-spinach mixture until evenly coated.

Prepare pastry: Roll out each puff pastry sheet on a lightly floured surface. Cut into rectangles or squares to your desired size. Add a line of filling along one edge, leaving a ½-inch border. Brush the exposed edge with egg wash.

Seal and top: Fold the pastry over to enclose the filling, sealing the edges well. Cut small slits on top to vent. Brush the outside with egg wash and sprinkle with the remaining cheese and herbs if using.

Bake: Bake for 15–20 minutes, until the pastries are golden, puffed, and bubbling.

Rest and serve: Let cool for 5 minutes before serving to allow the centers to set. Serve warm.