Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper. Warm the olive oil in a large skillet or Dutch oven over medium-high heat. After 90 seconds, when the oil is hot, add the onion and sauté for 4-5 minutes until lightly golden.
Add the garlic and chopped fresh mushrooms, thyme, coriander, and black pepper. Sauté for 8–10 minutes, stirring occasionally, until the mushrooms release their liquid and begin to brown.
Add the porcini mushrooms, red wine, and tamari. Simmer for 4 minutes until all of the liquid has either evaporated or absorbed into the mushrooms. Remove from heat and let cool in the freezer for 10 minutes.
Once the mushroom mixture has cooled, stir in the parsley, roasted chestnuts, walnuts, and panko breadcrumbs until evenly combined.
Working with the chilled but not frozen dough, spread out one sheet of puff pastry into a rectangle large enough to hold the filling with room to fold over the edges. Spoon the cooled mushroom mixture into the center and shape it into a log. Fold the pastry over the filling, sealing the edges tightly. Place the wrapped Wellington seam-side down on the prepared baking sheet.
Spread out the second sheet of puff pastry and use a lattice cutter to create a decorative top layer (or score it creatively with the tip of a sharp knife). Carefully drape it over the wrapped Wellington, trimming any excess pastry and pressing the edges to seal.
Completely optionally, use leftover strips of puff pastry to add braided accents to the top. Once braids have been made, stretch them slightly to extend the length of the Wellington and apply them to the surface where they will naturally stick.
In a small bowl, whisk together the plant-based milk, dijon mustard, and maple syrup. Brush this mixture evenly over the pastry surface.
Bake for 35–40 minutes, or until the pastry is deep golden and crisp. Allow the Wellington to rest for 10 minutes before slicing and serving.