Preheat the oven to 180°C/Gas Mark 4. Lay the Portobello mushrooms and stalks onto a baking tray and drizzle 1 tbsp olive oil, ensuring they are coated.

Bake in the oven until cooked. Remove from the oven and carefully pat with kitchen roll to remove excess liquid. Place on fresh kitchen roll to cool.

Place a large pan on the hob on medium heat and add 1 tbsp olive oil. Fry the onion and one crushed garlic clove for 5 minutes, then add the chopped mushrooms. Fry until cooked. Remove the mushroom mixture and spread out onto a baking tray to cool.

Add 1 tbsp olive oil to the pan. Add the spinach and a crushed garlic clove and fry until the spinach has wilted. Remove from the heat and put in a bowl to cool.

In a food processor, add the cooled mushroom mix plus the stalks from the Portobellos, chestnuts, nuts and tarragon. Add a pinch of salt and blitz to a paste. Remove and place to one side.

On a clean, lightly dusted work surface, unroll one pastry sheet. Spoon half the mushroom mixture down the centre of the pastry, leaving a border round the edge. Cover with half the spinach. Place three Portobellos along the centre and three more on top so they are stacked. Carefully cover with the remaining spinach and then the remaining mushroom mixture. Using a pastry brush, brush the edges with plant milk.

Unroll a sheet of pastry and place on top, carefully shaping with the sides of your hands. Score the pastry into diamonds (or any decoration you desire) and glaze with plant milk. In the oven, still at 180°C/Gas Mark 4, bake for 45 minutes or until golden brown.