Preheat the oven to 200°C.
Heat 1 tablespoon olive oil in a frying pan over medium heat. Add the onion and cook, stirring occasionally, for 5 minutes.
Add the garlic and mushroom and continue to cook until the mushroom has broken down and any liquid has evaporated.
Transfer the mushroom mixture to a food processor, along with the stock cubes, soy sauce, Worcestershire sauce, thyme or rosemary leaves and the liquid smoke (if using) and blitz to combine.
Place the mince in a large bowl and use a fork to mash the mustard, pepper and parsley into the mince. Stir in the mushroom mixture until well combined. Divide the mixture into quarters.
Cut the pastry sheets in half and place them in front of you. Evenly spoon the mince mixture along the long edge of each pastry sheet, then fold the pastry sheets over to enclose the filling and use a fork to crimp and seal the edges. Press the sausage rolls down on this crease to seal shut, then cut in half and stuff any filling that’s fallen out back into the open ends.
Whisk 4 teaspoons of olive oil and the soy milk in a small bowl. Using a pastry brush, brush the mixture over the top of each sausage roll and sprinkle with the sesame seeds and/or poppy seeds (if using).
Bake for 25–35 minutes, until golden.
Serve with tomato ketchup or your favourite tomato sauce.