Line a 23cm (9-inch) round pie dish with one half of the puff pastry (roll it out first if using a pastry block).

Preheat the oven to 160°C fan (350°F/Gas 4) and line a baking tray with parchment paper.

Quarter the mushrooms and spread them over the lined baking tray. Drizzle with 1 tablespoon of the oil, season lightly with salt and pepper and roast for 15 minutes. When the mushrooms are golden and slightly shrivelled, remove and set aside, reserving any juices.

Meanwhile, warm the remaining 2 tablespoons oil in a large frying pan. Add the onions and garlic and cook for 10 minutes over a medium heat, stirring occasionally, until softened. Reduce the heat to medium-low.

Remove the leaves from the rosemary and thyme by running your thumb and forefinger from the top to the base of the stalks (the leaves should easily come away). Finely chop the leaves, discarding the stalks. Add to the pan along with the sugar and cook for 10 more minutes, until the onions are golden.

Pour the Guinness into the pan, bring to a simmer, and cook for 10 more minutes to reduce the liquid. Reduce the heat to low and add the mushrooms and any juices from the tray. Add the flour, mustard and soy sauce and simmer gently for 15 to 20 minutes, stirring regularly. Taste and adjust the seasoning, adding more salt, pepper, mustard or soy sauce if you like.

Let cool slightly, then spoon the mushroom mixture into the pie dish lined with puff pastry.

Lightly dust a work surface with flour and roll out the remaining pastry until it is large enough to cover the top of the pie dish. Brush the rim of the dish with water and

lay the pastry over the top. Cut off the excess pastry and crimp the edge either by pinching it between your finger and thumb all the way around, or by pressing it against the dish with the back of a fork.

Melt the vegan butter in the microwave and use a pastry brush to brush it all over the pastry. Use a small sharp knife to cut a little cross in the centre so that steam can escape. Top the crust with a few rosemary sprigs to make it look fancy. Bake for 30 to 35 minutes, until the pastry is golden brown. Serve hot.