In a medium saucepan, bring the stock to a boil and add the soya mince. Cook for 15 minutes.
While the soya mince cooks, heat a large frying pan over medium heat and add the vegetable oil. Once hot, add the red onion and cook for a few minutes, making sure the onion doesn't burn. Add the garlic and cook for another few minutes.
Drain the soya mince and add it to the pan along with the finely cubed mushrooms and cook for around 5 minutes or until the mushrooms have started to give up a lot of moisture and they're lightly starting to brown.
Add the miso, rosemary, soy sauce, marmite, black pepper and balsamic vinegar. Stirring regularly, cook for a few minutes until the mushrooms are nicely browned.
In a small bowl, whisk together the corn starch and the water until smooth with no lumps. Add the corn starch slurry to the pan and stir until well combined. The mixture should start to clump together more.
Spread out the filling mixture on a baking tray and leave it to cool down for a minimum of 15 minutes.
While the mixture is cooling, preheat the oven to 180c.
Unroll the puff pastry and slice it in half length-ways. Once the filling is cool, use a dessert spoon to spread a line of the sausage mixture down the centre of each strip of pastry, leaving around a centimetre gap to the left and around 3cm gap to the right (see picture above).
Fold the dough over in half, covering the filling and seal the cut edge by pressing a fork along the length.
Slice each strip of pastry into 5 and arrange on a baking tray lined with baking parchment.
Whisk together the plant milk, agave syrup and oil then brush over the top of each sausage roll. Sprinkle with sesame seeds, then place in the oven and bake for 30-40 minutes, rotating the tray half-way through.
Remove from the oven and allow to cool for ten minutes before serving. These guys are best served hot, so if you're eating them the next day, stick them in the oven at 100c for 10 minutes before serving.