Step 1
Wipe the portobello mushrooms clean using moist paper towels. Remove the stems, then slice off the excess rim that curls over the gills, creating a flat surface so the mushrooms can sear. Reserve the stems and scraps for use in Step 2. Brush the caps on both sides with 3 tablespoons olive oil and season generously with salt and pepper. Heat 2 tablespoons olive oil in a large nonstick skillet over medium-high and cook the mushrooms, gill-side down, until caramelized, 5 minutes, then flip and cook another 5 minutes. Transfer to a wire rack, gill-side down, to cool.
Step 2
Prepare the mushroom filling: Separate and reserve any mushroom stems. (If using shiitake mushrooms, save their woody stems for another use.) Roughly chop about two-thirds of the mixed mushrooms, then working in batches, transfer the roughly chopped mushrooms to a food processor and pulse into small, ¼- to ½-inch pieces; transfer to a large bowl. By hand, finely chop the remaining mixed mushrooms and stems (including the reserved portobello mushroom stems and trimmings) into ¼-inch pieces; add them to the bowl.
Step 3
Prepare an ice bath in a large bowl. (If preparing the mushrooms in advance, you can simply let the mixture cool to room temperature, then refrigerate.) Wipe out the skillet. Working in two batches, warm ¼ cup olive oil over medium-high. Add half the mushrooms, shallots, garlic and rosemary, and season with salt and pepper. Cook, stirring occasionally, until caramelized and tender, about 10 minutes. Transfer to a medium bowl. Repeat with the remaining ¼ cup oil and the remaining mushrooms, shallots, garlic and rosemary. Return the first batch to the skillet. Add the port, soy sauce and thyme leaves and cook over medium-high, stirring occasionally, until the liquid evaporates, 3 to 4 minutes. Transfer the mixture back to the medium bowl and stir in the walnuts. Set the bowl over the ice bath to cool, stirring occasionally, at least 20 minutes.
Step 4
Prepare the onions: Wipe out the skillet, then melt the butter over medium heat. Add the onions, sugar, salt and pepper, and cook, stirring occasionally, 5 minutes. Add the cider and cook, stirring occasionally, until the liquid evaporates and the onions are caramelized, about 15 minutes. Stir in the vinegar, if using, then let cool.
Step 5
Heat the oven to 400 degrees. Place a large piece of parchment paper on your work surface and lightly dust it with flour. Unfold your thawed puff pastry on the parchment. Using a lightly floured rolling pin, roll the pastry out into a 13-by-16-inch rectangle. Transfer the parchment paper and puff pastry to a large sheet pan. Rotate as needed so that one of the 16-inch sides is closest to you. Arrange half the cooked mushroom mixture in a 4-by-10-inch strip in the center of the puff pastry, leaving a 1½-inch border at the ends. Arrange the caramelized onions in a 3-inch-wide strip on top. Lay the portobello mushrooms on top of the onions in a single line, stem-side down. (If the portobellos are too large, square off edges.) Spoon the remaining mushroom mixture on top, covering the portobello mushrooms, then gently pack the mushroom mixture to form an even layer on top, shaping it like a freeform meatloaf.
Step 6
Lift one side of the puff pastry over the filling to almost completely cover it. Brush the surface of the puff pastry covering the mushrooms with the beaten egg. Lift the remaining puff pastry flap over the egg-washed puff pastry, gently stretching it if need be to create a second layer of puff pastry on top, then gently press the top layer of pastry onto the lower layer to seal. Brush the insides of the short ends of the puff pastry and press to seal, tucking both sides underneath. Trim any parchment paper that extends beyond the sheet pan. (You can refrigerate the pastry for up to 1 day at this point.)
Step 7
Brush the exposed puff pastry on top with the remaining beaten egg. Decorate the top of the puff pastry as you like: Create a cross-hatch pattern by gently slicing through only the top layer of puff pastry in parallel lines, then cutting parallel lines in another direction. (Apply very little pressure, as you only want to cut through the top layer of puff pastry.) You can also slice small decorative vents (be sure to slice all the way through both layers of puff pastry), or top with additional strips or shapes made from egg-washed puff pastry. Bake until puff pastry is deep golden and flaky, 45 to 50 minutes. Let cool slightly on the baking sheet, about 10 minutes.
Step 8
While the Wellington bakes, prepare the optional port reduction: In a medium saucepan, heat the oil over medium. Add the shallot, garlic and peppercorns, and cook, stirring occasionally, 3 minutes. Add the port, stock and thyme; cook over medium-high until the sauce is glossy, 25 to 30 minutes. Strain the sauce; you should have about ½ cup. When ready to serve, reheat over medium, whisk in the butter, and season with salt and pepper.
Step 9
To serve the mushroom Wellington, cut it crosswise into 8 even slabs. (Each slab will include a pretty cross-section showcasing the halved portobello mushroom in the center; this is considered the presentation side.) Serve each piece presentation-side up. Pass with port reduction for drizzling on top.
