Making Parmesan Mushroom Tartlets is easy if you follow these step-by-step instructions:

1. Preheat the Oven: Preheat your oven to 400°F (200°C).
2. Prepare the Puff Pastry: Roll out the thawed puff pastry on a lightly floured surface. Cut into squares (about 3 inches) and place them in a greased mini muffin tin.
3. Sauté the Mushrooms: In a skillet, heat olive oil over medium heat. Add the chopped mushrooms, minced garlic, salt, and pepper. Sauté until the mushrooms are softened and moisture has evaporated (about 5-7 minutes).
4. Mix the Filling: In a bowl, combine the sautéed mushrooms, softened cream cheese, grated Parmesan cheese, and chopped parsley. Mix until well combined.
5. Fill the Pastry Shells: Spoon the mushroom mixture into each pastry shell, filling them generously.
6. Egg Wash: Brush the edges of the pastry with the beaten egg for a golden finish.
7. Bake: Place the muffin tin in the preheated oven and bake for 25-30 minutes, or until the pastry is golden brown and puffed.
8. Cool Slightly: Once baked, remove the tartlets from the oven and allow them to cool in the tin for a few minutes before transferring them to a wire rack.

These steps will guide you through creating the most incredible Parmesan Mushroom Tartlets effortlessly.