Step 1: Prepare the Crust

1. Preheat oven to 375°F (190°C).
2. Roll out puff pastry and place it in a tart pan or on a lined baking sheet.
3. Use a fork to prick the pastry lightly to prevent puffing.
4. Brush the edges with egg yolk for a golden crust.

Step 2: Sauté the Mushrooms

1. Heat olive oil or butter in a skillet over medium heat.
2. Add shallots and garlic, sauté for 1 minute until fragrant.
3. Stir in mushrooms, thyme, salt, and pepper, and cook for 5-7 minutes until mushrooms are soft and golden.

Step 3: Assemble the Tart

1. Spread a thin layer of Dijon mustard (optional) on the pastry.
2. Evenly spread the cooked mushrooms over the pastry.
3. Crumble goat cheese over the mushrooms.
4. Drizzle heavy cream or sour cream on top.
5. Sprinkle with Parmesan cheese for extra richness.

Step 4: Bake & Serve

1. Bake for 20-25 minutes, until the crust is golden and crisp.
2. Let cool for 5 minutes, then slice and serve warm.