In a food processor, pulse flour and salt to combine. Add cold, cubed butter and pulse until mixture resembles coarse crumbs with some pea-sized butter pieces remaining. Pro tip: Keep butter cold for the flakiest crust - even chill your flour if your kitchen is warm.

Gradually add ice water, 1 tablespoon at a time, pulsing until dough just comes together. Turn out onto a lightly floured surface, shape into a disk, wrap in plastic wrap, and refrigerate for at least 1 hour. Pro tip: Don't overwork the dough - a few flour streaks are perfectly fine and will create a tender crust.

Heat olive oil in a large skillet over medium-high heat. Add sliced mushrooms and cook without stirring for 3-4 minutes to develop a golden crust. Stir and continue cooking until mushrooms are deeply caramelized, about 8-10 minutes total. Pro tip: Don't crowd the pan - cook mushrooms in batches if needed to ensure proper browning.

Add chopped shallot and garlic to the mushrooms, cooking for 2-3 minutes until fragrant. Pour in white wine (if using) and cook until evaporated. Stir in fresh thyme, season with salt and pepper, then remove from heat and let cool slightly. Pro tip: Taste and adjust seasoning - well-seasoned mushrooms are key to a flavorful tart.

Preheat oven to 375°F (190°C). Roll chilled dough on a floured surface into a 12-inch circle. Transfer to tart pan, pressing gently into corners. Trim excess, leaving a 1-inch overhang, then fold under and crimp edges. Pro tip: Roll from center outward, rotating dough frequently to maintain a round shape.

Line pastry with parchment paper and fill with pie weights or dried beans. Blind bake for 15 minutes, remove weights and parchment, then bake 5-8 minutes more until lightly golden. Remove from oven and reduce temperature to 350°F (175°C). Pro tip: Prick the bottom with a fork before adding weights to prevent puffing.

In a large bowl, whisk together eggs, heavy cream, milk, grated Gruyère, chopped parsley, and nutmeg. Season with salt and pepper to taste. Pro tip: Strain the custard mixture for an ultra-smooth filling if you want restaurant-level perfection.

Spread the cooled mushroom mixture evenly in the pre-baked tart shell. Pour the custard mixture over the mushrooms, ensuring it reaches all corners but doesn't overflow. Pro tip: Place the tart pan on a baking sheet to catch any potential spills and make transfer easier.

Bake the assembled tart for 30-35 minutes, until the custard is just set and the top is lightly golden. The center should still have a slight jiggle when gently shaken. Pro tip: Don't overbake - the custard will continue to set as it cools, and you want that silky texture.

Cool on a wire rack for at least 15 minutes before removing from the tart pan. Garnish with fresh herbs if desired and serve warm or at room temperature. Pro tip: Run a thin knife around the edges before removing from the pan to prevent sticking.