Vegetarian Mushroom Wellington

Wipe the portobello mushrooms clean with moist paper towels. Remove the stems and slice off the excess rim that curls over the gills to create a flat surface for searing. Reserve stems and scraps for the mushroom filling.

Brush the portobello caps on both sides with 3 tablespoons of olive oil and season generously with salt and pepper. Heat 2 tablespoons of olive oil in a large nonstick skillet over medium-high heat. Cook the mushrooms, gill-side down, until caramelized (about 5 minutes), then flip and cook for another 5 minutes. Transfer to a wire rack, gill-side down, to cool.

Separate and reserve any mushroom stems. Roughly chop about two-thirds of the mixed mushrooms. 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. Finely chop the remaining mixed mushrooms and stems (including reserved portobello stems and trimmings) into ¼-inch pieces by hand; add them to the bowl.

Prepare an ice bath in a large bowl. Wipe out the skillet. Working in two batches, warm ¼ cup olive oil over medium-high heat. Add half the mushrooms, shallots, garlic, and rosemary; 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 of mushrooms to the skillet. Add the port, soy sauce, and thyme leaves. Cook over medium-high heat, stirring occasionally, until the liquid evaporates, about 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, for at least 20 minutes. (If preparing in advance, let cool to room temperature, then refrigerate.)

Wipe out the skillet. Melt the butter over medium heat. Add the onions, sugar, salt, and pepper; cook, stirring occasionally, for 5 minutes. Add the cider and cook, stirring occasionally, until the liquid evaporates and the onions are caramelized, about 15 minutes. Stir in the balsamic vinegar, if using, then let cool.

Heat the oven to 400°F (200°C). Place a large piece of parchment paper on your work surface and lightly dust it with flour. Unfold the 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, rotating as needed so a 16-inch side 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. Spoon the remaining mushroom mixture on top, covering the portobello mushrooms, then gently pack the mixture to form an even layer, shaping it like a freeform meatloaf.

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 pastry, gently stretching it to create a second layer on top, then gently press the top layer 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.)

Brush the exposed puff pastry on top with the remaining beaten egg. Decorate the top as desired: create a cross-hatch pattern by gently slicing through only the top layer of puff pastry, or slice small decorative vents. Ensure vents go through both layers.

Bake until the puff pastry is deep golden and flaky, 45 to 50 minutes. Let cool slightly on the baking sheet for about 10 minutes.

While the Wellington bakes, prepare the optional port reduction. In a medium saucepan, heat the oil over medium heat. Add the shallot, garlic, and peppercorns; cook, stirring occasionally, for 3 minutes.

Add the port, stock, and thyme sprigs. Cook over medium-high heat until the sauce is glossy, 25 to 30 minutes. Strain the sauce; you should have about ½ cup.

When ready to serve, reheat the sauce over medium heat, whisk in the butter, and season with salt and pepper.

To serve, cut the mushroom Wellington crosswise into 8 even slabs. Serve each piece presentation-side up. Pass with the port reduction for drizzling on top.