Step 1

Thaw 1 sheet vegan puff pastry according to package directions and keep refrigerated once thawed until ready to use. Meanwhile, arrange a rack in the middle of the oven and heat the oven to 425ºF. Line a baking sheet with parchment paper.

Step 2

Trim the stems from 2 portobello mushrooms and scrape out the gills with a spoon. Cut the portobellos into 1-inch thick strips and place on the baking sheet. Drizzle with 1 tablespoon olive oil, season with 1/2 teaspoon kosher salt and 1/2 teaspoon black pepper, and toss to combine. Spread into a single layer and roast, flipping the mushrooms halfway through, until shriveled and start to leak moisture, 12 to 15 minutes total. Transfer the portobellos to a large plate and set aside to cool. Reserve the parchment and baking sheet.

Step 3

Reduce the oven temperature to 400ºF. Place 2 (10-ounce) packages cremini mushrooms in a food processor fitted with the blade attachment. Pulse until coarsely chopped, 15 to 20 (1-second) pulses.

Step 4

Mince 3 garlic cloves. Finely chop 1 medium shallot, 1 tablespoon fresh thyme leaves, and 1 tablespoon fresh rosemary leaves. Coarsely chop 8 ounces oyster or shiitake mushrooms.

Step 5

Heat the remaining 2 tablespoons olive oil and 2 tablespoons butter in a large frying pan over medium-high heat. Add the shallot and garlic and cook until just softened, 2 to 3 minutes. Add the cremini and oyster mushrooms and cook until tender and all of their excess moisture is evaporated, 6 to 7 minutes more.

Step 6

Add the thyme, rosemary, 2 cups packed baby kale, remaining 1 teaspoon kosher salt, and remaining 1/2 teaspoon black pepper. Cook until the kale is wilted, 1 to 2 minutes. Remove from the heat and stir in 2 tablespoons Dijon mustard. If the mixture is still too wet, strain through a fine-mesh strainer and press down lightly to squeeze out the excess liquid. Transfer to the reserved baking sheet, spread into an even layer, and cool completely.

Step 7

Line a second baking sheet with parchment paper. Unfold the puff pastry on the parchment with a short side closer to you. Spoon half of the mushroom mixture down the center of the puff pastry in an even layer, leaving a 2-inch border on the top and bottom. Arrange the portobello mushrooms on top of the mixture in an even layer. Spoon the remaining mushroom mixture on top of the portobellos and press gently down to pack the mixture around the portobellos.

Step 8

Carefully fold both long sides of the puff pastry to meet over the center and pinch the edges together seal. Fold the ends over and pinch to seal so that the pastry completely wraps over the filling. Using the parchment paper, gently roll the Wellington over so that the seam is on the bottom.

Step 9

Using a paring knife, gently score the top of the puff pastry with a crosshatch pattern without cutting all the way through the pastry. Beat 1 large egg and 2 tablespoons water in a small bowl until combined, then brush the top and sides of the Wellington with the egg wash.

Step 10

Bake until golden brown and cooked through, 30 to 35 minutes. Let rest for at least 20 minutes before slicing and serving.