Pre-heat the oven to 180°C/350°F/gas mark 4.
Heat a large frying pan with a drizzle of olive oil over a medium heat. Add the diced onion and cook for 10 minutes until soft.
Add the garlic and thyme, stir for one minute, before adding all of the mushrooms. Stir-fry for around three minutes.
Use a large spoon to drain the excess liquid from the pan, before adding the chestnuts.
Add the sherry to the pan, and let it cook for five minutes, before pouring into a bowl to cool whilst you prepare the pastry.
Roll out the puff pastry onto a floured surface until it is about 80 cm long and is about the thickness of a 50p coin. Turn it landscape and cut in half down the middle so you have two 40 cm pieces. Cut each half longways, so you have four long strips of pastry with each one measuring approx. 40 x 12cm.
To the chestnut and mushroom mixture, add the breadcrumbs, pepper, salt and sage. Mix until well combined. You should be able to form a crumbly ball with the mixture. Split this ball into four equal sized balls. You now have one ball of mixture for each strip of pastry.
Take a strip of pastry and place onto a lightly floured work surface. Turn it landscape, and imagine a line down the centre of it (lengthways). Using a tablespoon, place small heaps of the mixture all down one half of the pastry, leaving a 1 cm gap at the edge. Using your thumb and index finger, lightly pinch and squeeze the mixture into a sausage shape, so it is an even size all the way down and there are no gaps.
Use a pastry brush to lightly dampen the 1 cm gap of pastry edge with vegan spread. Fold the other side of the pastry over the top of the mixture, and roll the whole thing over onto the damp edge, as one long sausage shape.
Brush the whole thing lightly with vegan spread, before using a sharp knife to cut into roughly 2 cm pieces. Repeat this process with the other three strips of pastry and filling. (To serve as part of a main meal instead of a snack, simply cut into larger rolls around 5 cm each.)
Place each onto an oven tray lined with baking paper. Sprinkle the tops with poppy seeds (if using) and cook in the middle of the oven for 20-25 minutes until golden brown.