Mushroom Wellington

Directions

1. Wash the portobello mushrooms by wiping clean with a damp paper towel. Remove the stems and slice off the excess rim that curls over the gills, creating a flat surface so the mushrooms can sear. Set aside the stems and scraps for use in step 2. Brush the caps on both sides with 3 tbsp of olive oil and season well with salt and pepper. Heat 2 tbsp of olive oil in a large nonstick skillet over medium-high heat and cook the mushrooms, gill-side down, until caramelized, about 5 minutes, then flip and cook another 5 minutes. Transfer to a wire rack to cool, gill-side down. 
2. Prepare the mushroom filling. Wipe the remaining mushrooms clean with a damp paper towel. Remove the stems and set them aside. Roughly chop about two-thirds of the mixed mushrooms, then, working in batches, transfer them to a food processor and pulse into small, ¼" to ½" pieces, and transfer to a large bowl. By hand, finely chop the remaining mixed mushrooms and stems (including the reserved mushroom stems and trimmings) into ¼" pieces and add them to the bowl. 
3. Prepare an ice bath in a large bowl (or if preparing the mushrooms ahead of time, you can skip the ice bath and let the mixture cool to room temperature before refrigerating). Wipe out the skillet you used to cook the portobellos and warm ¼ cup of olive oil over medium-high heat. 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 of olive oil, 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 heat, 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, for at least 20 minutes. 
4. Prepare the onions. Wipe out the same skillet, then melt the butter (or butter substitute) over medium heat. Add the onions, sugar, salt, and pepper, and cook, stirring occasionally, about 5 minutes. Add the cider and cook, stirring occasionally, until the liquid evaporates and the onions are nice and caramelized, about 15 minutes. Stir in the vinegar, if using, then let cool. 
5. Heat the oven to 400°F. 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, and using a lightly floured rolling pin, roll out the pastry into a 13" x 16" rectangle. Transfer the parchment paper and puff pastry to a large sheet pan. Rotate as needed so that one of the 16" sides is closest to you. Arrange half the cooked mushroom mixture in a 4" x 10" strip in the center of the puff pastry, leaving a 1½" border at the ends. Arrange the caramelized onions in a 3" 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 the edges). Spoon the remaining mushroom mixture on top, covering the portobello mushrooms, then gently pack the mixture to form an even layer on top, shaping it like a freeform meatloaf.   
6. Lift one side of the puff pastry over the filling to almost completely cover it. Brush the surface of the puff pastry that's covering the mushrooms with the beaten egg (or egg substitute). Lift the remaining puff pastry flap over the egg-washed puff pastry, gently stretching to create a second layer of puff pastry on top, then gently press the top layer of puff pastry onto the lower layer to seal. Brush the insides of the short ends of the puff pastry, then press to create a seal and tuck both sides underneath. Trim any parchment paper that extends beyond the sheet pan. (You can refrigerate the pastry for up to one day at this point.) 
7. Brush the exposed puff pastry on top with the remaining egg wash. Decorate the top of the puff pastry as you like: To create a crosshatch pattern, gently slice parallel lines through only the top layer of puff pastry, then cut parallel lines in another direction. You can also slice small decorative vents (be sure to slice through both layers of puff pastry). Or top your pastry with additional strips or shapes made from egg-washed puff pastry. Bake until puff pastry is deep golden and flaky, about 45 to 50 minutes. Let cool slightly on the baking sheet, about 10 minutes before you slice and serve it.