Lou Oates didn’t grow up celebrating Thanksgiving — she was born in London — but she got a crash course in the holiday when she came to the United States almost 15 years ago to work as a vegan-recipe developer for Whole Foods. Since then she has hosted big holiday feasts for her American husband and friends with dishes like this portobello Wellington based on a recipe her dad used to make. “Vegan entrées can get complicated, and I didn’t want to create a dish that would take six hours,” she says. “Ultimately people just want a nice vehicle for gravy!”
Make the Wellington: Preheat the oven to 350˚ F. Heat 1 tablespoon olive oil in a medium skillet over medium heat. Stir in the onion to coat. Spread evenly in the pan, reduce the heat to low and cook, stirring occasionally, until caramelized, 12 to 20 minutes. Set aside.
Spread the butternut squash on a parchment-lined baking sheet. Drizzle with 1 tablespoon olive oil and gently toss to coat. Sprinkle with salt and lay 5 thyme sprigs on top. Bake until the squash is tender and starting to caramelize, about 15 minutes.
Gently pull apart the maitake mushrooms into small pieces and place in a medium bowl. Add 2 tablespoons olive oil, season with salt and gently toss to coat. Spread in an even layer on a separate parchment-lined baking sheet. Top with 5 thyme sprigs and 5 rosemary sprigs and bake 15 minutes. When the vegetables are cool to the touch, gently remove the herb sprigs and discard.
Toast the hazelnuts on a separate baking sheet until fragrant, 5 to 8 minutes. (Be careful not to burn the nuts.)
Heat 1 teaspoon olive oil in a medium skillet over medium heat. Add the garlic and cook, stirring, until softened. Add the spinach, season with salt and cook, stirring frequently, until wilted, 3 to 4 minutes. Let cool slightly, then gently squeeze out any liquid over the sink.
In the same skillet, heat the remaining 1 teaspoon olive oil over medium heat. Add the portobello mushroom caps and sauté until they soften and release their liquid, 7 to 8 minutes. Transfer to paper towels to drain. Gently press to remove any excess liquid.
Chop the leaves from the remaining 2 thyme sprigs and 2 rosemary sprigs. In a medium bowl, mix the caramelized onions, butternut squash, spinach, hazelnuts, maitakes, chopped herbs and 1 teaspoon each salt and pepper. Mix well with a spoon or your hands.
Preheat the oven to 400˚ F. Unfold the puff pastry on the counter. Spread the vegetable mixture on the pastry in an even layer, leaving a 2-inch border on one short side. (You will have some of the vegetable mixture left over.) On the short side opposite the clean border, arrange the portobello caps dome-side up in a line, about 2 inches in from the edge, overlapping if necessary. Top the mushrooms with the rest of the vegetable mixture.
Gently roll up the puff pastry tightly, starting at the side with the portobellos and pushing in any filling that falls out. Place seam-side down on a parchment-lined baking sheet. Gently score a crosshatch pattern into the top of the pastry with a sharp knife; brush with the vegan egg wash. Bake until golden brown, 15 to 20 minutes.
Meanwhile, make the gravy: Heat the olive oil in a medium saucepan over medium heat. Add the shallots, reduce the heat to medium low and cook, stirring, until they start to caramelize, 8 to 10 minutes. Stir in the flour and cook, stirring, until light golden brown, about 4 minutes. Slowly add the vegetable stock and red wine, whisking constantly so that no lumps remain. Increase the heat to medium high and cook, stirring, until the gravy thickens, 2 to 3 minutes. Add the tamari and sugar and season with salt and pepper. Slice the Wellington and serve with the gravy.