Making Thin Mushroom Tart is straightforward and fun. Follow these steps to create this delicious dish:

1. Preheat the Oven: Set your oven to 400°F (200°C).
2. Prepare the Mushrooms: In a skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent.
3. Add Garlic and Mushrooms: Stir in the garlic and sliced mushrooms. Cook until the mushrooms are tender and most of the liquid has evaporated, about 5-7 minutes.
4. Season: Add thyme, salt, and pepper to the mushroom mixture, stirring to combine. Remove from heat and let cool slightly.
5. Roll Out the Pastry: On a floured surface, roll out the puff pastry into a rectangle, about ¼ inch thick. Transfer it to a baking sheet lined with parchment paper.
6. Create the Crust: Use a sharp knife to score a border around the edge of the puff pastry, about 1 inch in from the edge. Be careful not to cut all the way through.
7. Mix the Filling: In a bowl, whisk together the sour cream, egg, and Parmesan cheese (if using). Add the cooled mushroom mixture and stir to combine.
8. Fill the Tart: Pour the mushroom filling into the center of the pastry, spreading it evenly within the scored border.
9. Bake: Place in the preheated oven and bake for 25-30 minutes, or until the pastry is golden brown and crisp.
10. Garnish and Serve: Remove from the oven, let cool slightly, and garnish with fresh parsley before serving.

With these instructions, you can effortlessly create a Thin Mushroom Tart that is beautiful and delicious!