1. Make the Pastry (If Making From Scratch):

• In a food processor, combine the flour and salt. Add the cold butter and pulse until the mixture resembles coarse crumbs.
• Gradually add ice water, one tablespoon at a time, and pulse until the dough begins to come together. Be careful not to overmix.
• Turn the dough out onto a floured surface and gently knead it together into a ball. Flatten into a disc, wrap in plastic wrap, and refrigerate for at least 30 minutes to firm up.

2. Preheat the Oven:

• Preheat your oven to 375°F (190°C).

3. Prepare the Filling:

• Heat olive oil or butter in a large skillet over medium heat. Add the sliced leeks and sauté for 5-7 minutes, until softened and slightly caramelized. Stir occasionally to avoid burning.
• Add the sliced mushrooms and garlic to the pan and cook for another 5-7 minutes, until the mushrooms release their moisture and become golden brown. Season with salt, pepper, and thyme. Remove from heat and allow to cool slightly.

4. Roll Out the Dough:

• On a floured surface, roll out the chilled dough into a circle about 12 inches in diameter (or the size of your tart pan). Carefully transfer the dough to a greased tart pan or pie dish, gently pressing it into the corners and up the sides. Trim any excess dough hanging over the edge.

5. Assemble the Tart:

• Once the mushroom and leek mixture has cooled slightly, spread it evenly over the prepared pastry crust.
• In a bowl, whisk together the eggs and cream. Stir in the grated cheese, if using. Pour the egg mixture over the mushrooms and leeks, spreading it out to ensure the filling is evenly distributed.

6. Bake the Tart:

• Bake the tart in the preheated oven for 30-35 minutes, or until the filling is set and the top is lightly golden. A knife inserted into the center should come out clean.
• If the crust edges start to brown too quickly, cover them with foil or a pie shield to prevent burning.

7. Cool and Serve:

• Allow the tart to cool for about 10 minutes before slicing. Garnish with freshly chopped parsley and serve warm or at room temperature.