Clean 2 portobello mushrooms and 400 g chestnut or wild mushrooms (wash or wipe with a dry cloth).
Using a medium saucepan over medium heat, melt 1 tablespoon vegan butter. Put the portobello mushrooms top down and cook for 2 minutes, or until seared and golden brown. Repeat the process for the other side. Remove from the pan.
In the same pan, add your chopped mushrooms, 3 cloves garlic (chopped), 1 medium onion (chopped), 1 teaspoon French wholegrain mustard, 1 teaspoon dark soy sauce, ½ teaspoon tomato paste, ½ teaspoon salt, and cracked black pepper. If your pan isn't big enough, cook in two batches. Cook dry (no fat) until all the water from the mushrooms has evaporated. At this point, add 2 tablespoons vegan butter and cook until caramelised — around 3 minutes. Season with 1 sprig fresh rosemary and 4 sprigs fresh thymeurn off the heat, and cool.
Preheat your oven to 200℃ (400℉).
Take 300 g ready-rolled puff pastry (I use ready-rolled. Alternatively, roll it out to around 16x30cm) and spread a layer of cooked chopped mushrooms in the middle, then add the portobello mushrooms, and another layer of chopped mushrooms atop. Fold the pastry over on all sides. Carefully turn upside down onto a parchment-lined baking sheet.
Assemble the mushroom wellington by spreading a layer of the chopped mushrooms, adding the portobello mushrooms, and then another layer of chopped mushrooms atop. Fold over the pastry on all sides, completely covering the mushrooms.
Mix your vegan egg wash by combining 2 tablespoon soy milk with 1 tablespoon vegan butter (melted). Using a pastry brush, brush over the sides and top of your vegan mushroom wellington.
Use a knife to score a pattern in the wellington. Be careful not to cut through the pastry. Add a hole on top for steam to escape.
Bake for around 45 minutes — or until golden brown and cooked through on all sides. It may take slightly longer, depending on your oven.
Notes
* As long as you have more than 300g puff pastry, it's fine. If you have excess, you can use the offcuts to decorate the wellington. Frozen puff pastry also works fine; I often store mine in the freezer and defrost it in the fridge, 6 hours before baking. Most puff pastry is naturally vegan, but always check the ingredient lists.