Prepare the Portobello Mushrooms:

• Clean and remove the stems. Slice off excess rim for a flat surface.
• Brush both sides with 3 tablespoons olive oil, season with salt and pepper.
• Heat 2 tablespoons oil in a skillet over medium-high. Sear mushrooms, gill-side down, for 5 minutes, then flip and cook another 5 minutes.
• Transfer to a wire rack to cool.

Make the Mushroom Filling:

• Roughly chop two-thirds of the mixed mushrooms, then pulse in a food processor until finely chopped.
• Finely chop the remaining mushrooms and reserved portobello stems.
• Prepare an ice bath.
• In a skillet, heat ¼ cup oil over medium-high. Cook half the mushrooms, shallots, garlic, and rosemary for about 10 minutes.
• Repeat with remaining oil and ingredients, then combine both batches.
• Add port, soy sauce, and thyme. Cook until the liquid evaporates (3-4 minutes).
• Stir in toasted walnuts. Cool over an ice bath for 20 minutes.

Caramelize the Onions:

• Melt butter in a skillet over medium heat. Add onions, sugar, salt, and pepper.
• Cook for 5 minutes, then add cider and simmer until liquid evaporates (about 15 minutes).
• Stir in balsamic vinegar (if using), then let cool.

Assemble the Wellington:

• Preheat oven to 400°F (200°C).
• Roll out puff pastry into a 13″x16″ rectangle on floured parchment.
• Place half the mushroom filling in a 4″x10″ strip in the center.
• Layer caramelized onions over mushrooms, then top with portobello mushrooms.
• Cover with remaining mushroom mixture, shaping it into an even loaf.

Wrap the Wellington:

• Fold one side of the pastry over the filling. Brush with egg wash.
• Fold the second side over, pressing to seal.
• Seal and tuck in the short ends. Brush the entire pastry with egg wash.
• (Optional: Refrigerate for up to 1 day before baking.)

Bake:

• Place on a parchment-lined baking sheet. Bake at 400°F (200°C) for 30-35 minutes, until golden brown.
• Let rest 10 minutes before slicing.

Optional Port Reduction:

• Heat olive oil in a saucepan. Sauté shallots, garlic, and peppercorns.
• Add port and simmer until reduced by half.
• Add vegetable stock and thyme. Reduce again.
• Strain, then whisk in butter. Season with salt and pepper.