Prepare the portobellos: Preheat oven to 400°F (200°C). Brush the portobello mushrooms with 1 tablespoon olive oil, season with salt and pepper, and roast gill-side down for 15 minutes. This removes excess moisture and concentrates their flavor. Once done, set aside to cool.
Make the duxelles: Heat 2 tablespoons olive oil in a large skillet over medium heat. Add onions and cook until translucent (about 5 minutes). Add garlic and cook for another minute until fragrant.
Cook the mushroom mixture: Add the chopped mixed mushrooms to the skillet with the thyme and rosemary. Cook for 10-15 minutes, stirring occasionally, until all moisture has evaporated and the mixture is deeply browned. This concentration of flavor is essential for a rich filling.
Season the filling: Stir in soy sauce, balsamic vinegar, and nutritional yeast. Cook for another 2 minutes until everything is well combined. Transfer to a bowl and let cool for 10 minutes.
Complete the filling mixture: Once cooled slightly, mix in the breadcrumbs and toasted walnuts. The mixture should hold together when pressed but not be too wet. Adjust seasoning with salt and pepper.
Assemble the wellington: Roll out the puff pastry on a floured surface to about 14×12 inches. Spread a thin layer of Dijon mustard down the center, leaving a 2-inch border all around.
Layer the components: Spread half the mushroom mixture down the center of the pastry. Place the roasted portobello mushrooms on top, then cover with remaining mushroom mixture, creating a log shape.
Wrap the pastry: Carefully fold the shorter sides of the pastry over the ends of the filling, then fold the longer sides over the top, overlapping slightly. Pinch the seams to seal well. Flip the wellington over so the seam is on the bottom.
Score and glaze: Transfer to a parchment-lined baking sheet. Mix plant milk with maple syrup and brush over the pastry. Using a sharp knife, score diagonal lines across the top of the pastry for decoration and to allow steam to escape.
Bake to golden perfection: Bake for 35-40 minutes until the pastry is puffed and deeply golden. Let rest for 10 minutes before slicing to allow the filling to set.