Caramelize the Onions

1. Heat olive oil and unsalted butter in a large skillet over medium heat. Add onions, salt, pepper, and brown sugar, stirring to coat.
2. Reduce heat to medium low and cook for 20-25 minutes, until onions are thoroughly browned and softened.
3. Pour in apple cider vinegar and scrape up any browned bits on the bottom of the pan. Cook another 2-3 minutes, until liquid has evaporated. Remove onions to a plate lined with a paper towel and let cool completely.

Make the Filling

1. While caramelizing the onions, preheat oven to 400˚F. Prepare a baking sheet with parchment paper. Brush both sides of the portobello mushrooms with 1 teaspoon of the olive oil, and sprinkle with 1 teaspoon of the salt and 1/2 teaspoon of the pepper.
2. Place on the prepared baking sheet, stem side down, and bake for 10 to 15 minutes, until they just start to release their juices and their skin begins to wrinkle. Remove from heat and set aside to cool completely.
3. While baking the portobello mushrooms, roughly dice half of the mixed mushrooms and set aside. Using a food processor, pulse the remaining half into small pieces.
4. Heat remaining olive oil in a large skillet over medium high heat. Add shallots and garlic and cook for 1 to 2 minutes until fragrant.
5. Add in hand chopped mushrooms, 1 teaspoon of the salt, and 1/2 teaspoon of the ground black pepper and cook for 5 minutes, stirring frequently, until slightly softened.
6. Pour in half of the red wine and scrape up any browned bits on the bottom of the pan.
7. Stir in the remaining pulsed mushrooms, spinach, fresh herbs, salt and pepper. Cook another 4 to 5 minutes, until spinach is wilting and mushrooms have released some of their juices.
8. Pour in remaining red wine, soy sauce, and balsamic vinegar, scraping up any browned bits on the bottom of the pan. Cook another 2 to 3 minutes, until liquid has evaporated.
9. Remove from heat. Stir in walnuts. Add salt and pepper to taste. Remove to a shallow container or a baking sheet to cool completely.

Assemble the Wellington

1. Dust a surface lightly with all purpose flour. Gently unfold/unroll one thawed (but chilled) puff pastry sheet.
2. Prepare a plate with a silpat or parchment paper. Using a lattice cutter, stencils, cut outs, etc. make dough cut outs to decorate the top of the wellington log as desired. Place on the prepared plate. When done, put in the fridge to chill until ready to use.
3. Preheat oven to 350˚F. Prepare a baking sheet with parchment paper. Gently unfold/unroll the remaining puff pastry sheet. Roll out into a 13X16 inch rectangle and place on the prepared baking sheet.
4. Leaving a two inch border on the top and bottom of the pastry, spread half of the cooled mushroom mixture in an even layer about 4 inches wide along the center of the pastry rectangle.
5. Next, evenly spread the cooled, caramelized onions on top of the mushroom mixture.
6. Gently lay the cooled portobello mushrooms in a single layer on top of the caramelized onions, cutting off the edges of the portobellos as necessary to avoid overlap.
7. Top with another even layer of the remaining mushroom mixture.
8. Finally, sprinkle with the goat cheese crumbles.
9. Gently stretch and fold one side of the puff pastry up and over the filling. Brush lightly with egg wash.
10. Gently stretch and fold the other side of the puff pastry up and over the filling. Pinch the ends together and cut off excess.
11. Using a sharp paring knife or bread lame, score the top layer of the pastry in a cross hatch pattern (or whatever pattern you desire), being careful to only cut through through top layer, leaving the layer underneath uncut.
12. Brush all over with egg wash. Top with chilled cut outs and lattice work as desired. Brush with any remaining egg wash.
13. Place in the preheated oven on the center rack. Bake for 35 to 45 minutes (may take more or less time depending on your oven), until the crust is deeply golden and flaky. Remove from heat and let rest for 10 minutes.
14. Garnish with any remaining fresh herbs and a pinch of finishing salt. Slice and serve warm with a drizzle of your favorite vegetarian gravy!