Part 1: Preparing the Creamy Mushroom Filling

1. Sauté the Mushrooms: The key to a non-soggy, intensely flavored filling is to cook the mushrooms properly. Heat 1 tablespoon of butter and 1 tablespoon of olive oil in a large, heavy-bottomed skillet or Dutch oven over medium-high heat. Once the butter is melted and foaming, add half of the sliced mushrooms in a single layer. Don’t overcrowd the pan, as this will cause the mushrooms to steam instead of brown. Cook for 5-7 minutes, stirring only occasionally, until they release their moisture and begin to turn golden brown and caramelized on the edges. Remove the first batch with a slotted spoon and set them aside in a bowl. Repeat the process with the remaining 1 tablespoon of butter, 1 tablespoon of olive oil, and the rest of the mushrooms. This browning step, known as the Maillard reaction, is crucial for developing deep, savory flavor.
2. Cook the Aromatics: Reduce the heat to medium. To the same skillet, add the sliced leeks. Cook, stirring often, for about 8-10 minutes until they are very soft and slightly sweet. If the pan seems dry, you can add a small splash of water or broth to prevent sticking. Add the minced garlic and fresh thyme leaves and cook for another minute until fragrant. Be careful not to burn the garlic.
3. Deglaze the Pan: If using, pour in the white wine. Use a wooden spoon to scrape up any browned bits (the “fond”) from the bottom of the pan. This is where a huge amount of flavor is concentrated. Let the wine simmer and reduce by about half, which should take 2-3 minutes.
4. Create the Roux and Sauce: Sprinkle the all-purpose flour over the cooked leeks and garlic. Stir continuously for about 1-2 minutes to cook out the raw flour taste. This flour and fat mixture is a “roux,” which will thicken our sauce. Slowly pour in the warmed vegetable broth while whisking constantly to prevent lumps from forming. The mixture will begin to thicken.
5. Simmer and Combine: Bring the sauce to a gentle simmer. Once it has thickened slightly, stir in the heavy cream and the soy sauce. Return the cooked mushrooms to the skillet. Stir everything together to combine.
6. Final Seasoning: Let the filling simmer gently for 5-10 minutes, allowing the flavors to meld and the sauce to reach a thick, gravy-like consistency. It should be thick enough to coat the back of a spoon. Remove from the heat and stir in the chopped fresh parsley. Season generously with salt and freshly ground black pepper to your taste. Allow the filling to cool for at least 20-30 minutes. Putting a hot filling under puff pastry will cause the butter in the pastry to melt prematurely, resulting in a soggy bottom crust.

Part 2: Assembling and Baking the Pie

1. Preheat and Prepare: Preheat your oven to 400°F (200°C). Position a rack in the lower third of the oven.
2. Fill the Pie Dish: Pour the cooled mushroom filling into a 9-inch pie dish or a similar-sized oven-safe skillet or baking dish. Spread it out evenly.
3. Top with Puff Pastry: Gently unfold your thawed puff pastry sheet on a lightly floured surface. If needed, roll it out slightly so it’s about an inch larger than your pie dish. Carefully drape the pastry over the filling.
4. Trim and Crimp: Trim the excess pastry around the rim of the dish, leaving about a 1/2-inch overhang. You can tuck this overhang under itself and then use your fingers or a fork to crimp the edges for a decorative seal.
5. Egg Wash and Vent: Brush the entire surface of the pastry evenly with the egg wash. This is what will give your pie a beautiful, shiny, deep golden-brown finish. Using a sharp paring knife, cut 3-4 small slits in the center of the pastry. This allows steam to escape during baking, preventing the crust from becoming soggy.
6. Bake to Perfection: Sprinkle the top of the pie with a pinch of flaky sea salt. Place the pie dish on a baking sheet to catch any potential drips. Bake in the preheated oven for 25-35 minutes, or until the pastry is puffed up, deeply golden brown, and the filling is hot and bubbling through the vents.
7. Rest and Serve: Once baked, remove the pie from the oven and let it rest for at least 10-15 minutes before slicing and serving. This resting period is crucial as it allows the filling to set up, making it easier to slice cleanly.