Heat the olive oil in a large pan over medium heat and add the onion, celery and carrot. Sauté for 5 minutes, then add the garlic and cook for another 3 minutes or until all the vegetables are tender.
Add the chopped mushrooms and sauté for 2 minutes and then add the lentils, rosemary, Dijon mustard, nutritional yeast, red miso paste and apple cider vinegar. Cook the mixture down for 5 minutes, until most of the moisture has evaporated. Season with salt and pepper and add the chopped spinach. Remove the mixture from the heat and transfer it to a large bowl to cool completely to avoid soggy rolls.
To prepare the sausage rolls, preheat the oven to 400°F (200°C) and line two baking trays with greaseproof paper. Remove the puff pastry from the freezer to thaw for 10 minutes. Stir the breadcrumbs into the “sausage” mixture. The mixture should hold together when squeezed; add some more breadcrumbs if needed.
Slice the thawed pastry sheets in half lengthwise. Take one-half and lay it with the long side closest to you. Wet the sides and the top edge of the pastry with water spoon 3 to 4 tablespoons (45 to 60 g) of the filling onto the pastry edge closest to you. Shape the filling into a log. Gently but firmly roll the pastry and filling away from you to form a large roll. Press the edges to seal.
Slice the roll into 4 or 5 smaller rolls and make an incision into the top of each. Brush each roll with the dairy-free milk, sprinkle with sesame seeds andplace on the lined baking tray seam side down. Repeat with the remainingfilling and pastry.
Bake the rolls in the oven for 15 to 20 minutes or until golden and puffed.