Heat the oil in a large frying pan over a medium/high heat, then add the mushrooms and onion and cook, stirring often, until all the moisture from the mushrooms evaporates. It could take up to 10 minutes.
Add the garlic, chestnuts, spinach, soy sauce, sage, salt and pepper and cook, stirring often, for a further 4 minutes or until the moisture evaporates and the mixture starts to stick to the bottom of the pan.
Allow the mixture to cool slightly, then press into a log shape, roll in cling film and refrigerate for at least 30 minutes. NOTE: Cooling it stops the pastry from getting soggy. If you don't have time to wait, it will still be delicious.
Heat the oven to 400F / 200C. Line a baking sheet with baking paper.
Roll out the sheet of puff pastry. Unwrap the filling from the clingfilm and place it on the pastry. Brush a little water around the edges of the pastry. Fold the pastry around the filling (trim any excess pastry) and press the seams to seal.
Place the wellington with the seam side down on the baking tray. Use a sharp knife to score the pastry diagonally.
Brush the wellington with a little dairy-free milk, place in the oven and bake for 30 minutes or until the pastry is golden and flaky.
Delicious served warm with vegan gravy and roasted vegetables.
Chop the mushrooms in various shapes and sizes to get a nice texture.
When cooking the mushroom mixture, cook until the pan is dry. You want as much moisture to evaporated as possible to ensure your pastry doesn't get soggy.
There's no need to add salt, since the soy sauce is salty enough.
Be sure to let your filling chill completely before wrapping it in the puff pastry. This will ensure that you don't get a soggy bottom.
All puff pastry should be vegetarian, but double-check the package if you want vegan.
I use store-bought puff pastry. But you could make your own.
For a vegetarian wellington, you can use an egg wash to get your pastry golden and shiny.
For a vegan wellington, don't use the egg wash, instead try vegan milk mixed with a little olive oil, or just vegan milk.
Storage and Freezing
The cooked mushroom wellington will keep in the fridge for up to 3 days. Reheat as needed.
You can freeze the cooked wellington. Allow it to cool completely, then cook from frozen for approximately 40 minutes.
Make-Ahead WellingtonFor best results, you can make the filling ahead (up to 3 days) and keep it in the fridge. Then simply wrap it in the pastry and cook before serving.