Preheat oven to 400 degrees F. Generously coat a muffin tin, mini or full-size, with cooking spray and set aside.
Gently unfold the puff pastry sheet and use a pizza wheel or kitchen scissors to cut it into squares. (16 squares if using a mini muffin tin; 9 if full-sized.) Gently press a square into each muffin well.
Spoon a bit of sauce into each well, just enough to cover the bottom. Sprinkle a bit of cheese on top of the sauce, then fill with pepperoni and/or mushrooms. Sprinkle a couple extra shards of mozzarella and a pinch of Italian seasoning on top.
In a small bowl, whisk together the egg with 1 tablespoon water to form an egg wash. Brush the wash lightly over the exposed corners of puff pastry. (This is optional; it just gives the pastry a nice golden sheen.)
Bake for 15-20 minutes, until cheese is melted and puff pastry is golden brown. Remove from the oven, let cool briefly, and pop bites out of the muffin tin. Serve hot, with more sauce for dipping, if desired. Enjoy!
You want the puff pastry sheet to be thawed and pliable, but still cold. If the pastry is not still cool to the touch after cutting and pressing the squares into the muffin tin, pop it into the fridge for 10-15 minutes before baking for the best (flakiest, most delicious) result.
I used Pepperidge Farms Puff Pastry sheets, and this is what the 16/9 square recommendations are based on. If you use a different brand, you may need to adjust. Just take a look and aim for your puff pastry squares to cover most of the muffin well when pressed in and up along the sides.