Preheat the oven to 400°F (200°C).
Sauté the onions and garlic in a pan until translucent.
Add the mushrooms and cook until moisture evaporates.
Stir in the chopped chestnuts, thyme, soy sauce, and breadcrumbs.
Season with salt and pepper, adjusting to your taste.
Roll out the puff pastry and place the filling in the center.
Fold the pastry over the filling and seal the edges.
Brush the top with plant-based milk and cut slits for steam.
Place the Wellington on a baking sheet and bake for 25-30 minutes.
Let it cool slightly before slicing.