Heat olive oil in a large pan over medium heat. Add finely chopped onion and sauté until softened, about 5-7 minutes.

Add minced garlic and cook for another minute until fragrant.

Stir in the finely chopped mixed mushrooms. Cook, stirring occasionally, until all the liquid has evaporated and the mushrooms are nicely browned, about 10-15 minutes.

Add the chopped chestnuts, toasted walnuts, fresh sage, and fresh thyme to the pan. Pour in the vegetable broth and simmer for 5 minutes.

Stir in the breadcrumbs until the mixture thickens. Season generously with salt and black pepper to taste. Remove from heat and let the mixture cool completely (this is crucial for crisp pastry).

Preheat your oven to 200°C (400°F). Line a baking tray with parchment paper.

Unroll the vegan puff pastry sheet onto the prepared baking tray. Spoon the cooled mushroom and chestnut mixture along the center of the pastry, shaping it into a compact log.

Carefully fold one long side of the pastry over the filling, then fold the other long side over, sealing the edges firmly to create a neat parcel. Trim any excess pastry from the ends and press to seal.

(Optional) Brush the top of the Wellington with plant milk for a golden finish. Using a sharp knife, score the top of the pastry with decorative shallow cuts, being careful not to cut through to the filling.

Bake for 35-45 minutes, or until the puff pastry is beautifully golden brown and puffed up.

Remove from the oven and let the Wellington rest for 10 minutes before slicing and serving. Enjoy your festive vegan main!