Preheat your oven to 200°C (400°F).

In a large skillet, heat olive oil over medium heat.

Add the chopped onion and garlic; sauté until translucent.

Stir in the mixed mushrooms and cook until they release their moisture and are golden brown.

Add the cooked chestnuts, thyme, soy sauce, nutritional yeast, salt, and pepper; mix well and cook for an additional 2-3 minutes.

Remove from heat and let the filling cool slightly.

Roll out the puff pastry on a lightly floured surface to fit your baking tray.

Place the mushroom and chestnut filling in the center of the pastry, shaping it into a log.

Fold the pastry over the filling, sealing the edges well.

Place the Wellington on a baking sheet lined with parchment paper.

Brush the pastry with plant-based egg wash for a golden finish.

Bake for 25-30 minutes or until golden brown and puffed up.

Remove from the oven and let the Wellington rest for 5-10 minutes before slicing.