Add olive oil, chopped onion, crushed garlic, dried rosemary, coriander powder, dried thyme and oregano to a pot and sauté until the onions are softened.
Add in the sliced mushrooms and soy sauce and sauté until the mushrooms are softened.
Add cashews, water and salt to your blender jug and blend until smooth.
Add the cashew cream to the mushrooms and heat everything together until it reaches a simmer. It will thicken as it heats and create a thick, creamy sauce.
Turn off the heat and leave the sauce to cool as it will thicken more as it cools.
When the sauce has cooled, preheat the oven to 390°F (200°C).
Line a baking tray with parchment paper. Lie the sheet of puff pastry down on top of the parchment paper. You may have to trim your puff pastry sheet slightly to fit your baking tray.
Add the mushroom filling along one side of the puff pastry, leaving a margin at the edge so that when you fold the pastry over you will be able to seal it along the sides (see our photos above to see this in action).
Fold the puff pastry over the top of the mushroom filling, sealing it along the margin you left along the edge. Use a fork to make ridges in the pastry at the edges and help to seal it in even better.
Brush the top of the pastry with melted vegan butter and cut diagonal slashes across the top of the pastry.
Place into the oven and bake for 25 minutes or until golden brown and crispy.
Allow to cool for 5 minutes before serving. Top with a sprig of fresh rosemary and serve.