Add 2 tablespoons of olive oil to a large saucepan over a medium heat. After a few minutes, when the oil has warmed, add the chopped onion and let it become golden and translucent, 2-3 minutes. Next add the chopped celery and leek, stirring again and allowing to cook together for 5-8 minutes to caramelise. You may need to turn the heat down if it seems like it’s catching on the bottom.
Add the chopped mushrooms and garlic and stir again. Turn the heat up slightly whilst stirring occasionally for a further 10 minutes, making sure the garlic doesn’t burn while the mushrooms lose their water and reduce, becoming dark brown and golden.
Reduce the heat and add all the seasoning; the walnuts, fennel seeds, salt and pepper to taste, maple syrup, balsamic vinegar, nutritional yeast if you have it, if not a teaspoon of yeast extract such as Marmite works well for extra umami.
Add the fresh herbs, parsley and thyme (rosemary also works well here if it’s very finely chopped). If you can’t find fresh thyme, a heaped teaspoon of dried thyme would also work.
Next add the breadcrumbs or stuffing mix and vegetable stock. Start by adding half the stock and stir to bind all the ingredients together. You may only need ¼ cup of stock if the mixture appears to be binding well (depending on how wet the mushrooms become), if not, add the full ½ cup and mix well. Take off the heat and leave to cool while you roll out the dough.
Preheat the oven to 190°C/375°F/gas mark 5 and have a large oven tray ready which is suitable for the log.
Take 500 g of puff pastry and flour a clean work surface. Working with a floured rolling pin, roll the puff pastry to about ½ cm in even thickness, if you like thicker pastry then leave it just slightly thicker than this. You are aiming for a rectangular shape, with a rough 3×2 ratio so bear this in mind. Once the pastry is rolled to its thickness, lightly flour again to help any sticking and begin to add the filling.
Pile the filling along the length of the rectangle in the centre, ensuring it is evenly spread out and well-compacted. The pastry on either side of the filling will be folded up to cover it so make sure you’re not making your filling too large or too spread out. Make sure the filling is piled right up to the edges at either long end otherwise the end slice won’t hold up once it’s cooked. Brush the long edges with your plant-based milk of choice.
At this point you can either simply fold into the centre either side of pastry so that the mushroom roll is covered or you can use a sharp knife to create an easy plait effect that looks great as a centrepiece.
For the plait, leaving 2-3 inches either side of the filling for the pastry to come up the sides of the log centre, take a sharp knife and make incisions from the filling out to the edge, being careful not to squish the pastry. Repeat the cuts in a diagonal line across the long edge of the pastry, leaving an inch or so between the incisions and leaving 2-3 inches untouched at either long end before beginning. Repeat on the other side of the log with the cuts going in a symmetrical direction so that they create shallow ‘V’s on either side of the filling.
Now to fold! Start by securing in place the end pieces by taking one side and bringing it into the centre and then the either side folded over the top. As you begin the folding, the pastry will want to all lift together so you’ll need to plait the pastry all in one go. Fold one piece over the other, overlapping them and repeating this the whole way along the log to form the plait. Don’t pull the pastry too much as it will simply shrink back and crack once it’s in the oven – a few gaps are fine and will add to the rustic look of the log! Be confident with the plaiting and keep moving along the log. Once satisfied with the finished log, firmly tuck the filling in to each end to ensure it won’t fall out and brush the outside of the log with your milk of choice or even a flavourless vegetable oil.
Now carefully slide the whole log onto a greased baking sheet and add to the preheated oven at 190°C/375°F/gas mark 5 for half an hour. If it seems too golden after 20 minutes and look like it may burn, add a sheet of foil to the top to stop the browning. Similarly, leave it near the top of the oven for longer if your pastry hasn’t become fully golden.
Slice and serve hot! Reheat any extra in an oven to keep the crispness of the pastry, otherwise can be kept for 4-5 days in the fridge and enjoyed alongside festive leftovers.