Preheat oven to 180 degrees celsius fan-forced (355 fahrenheit).
Heat the olive oil in a large frying pan over medium/high heat.
Saute the diced onion, garlic and herbs for 3 minutes (or until the onion is softened).
Add the mushrooms and cook for 5 minutes (until softened and all of the liquid has evaporated).
Add the grated sweet potato and carrot and cook for a further 3 minutes.
Stir through the lentils and soy sauce. Season with salt and pepper. Pour into a bowl and allow to cool slightly. Stir through the grated cheese.
Cut each sheet of defrosted puff pastry in half.
Whisk the egg lightly. Using a pastry brush, lightly brush the egg mixture down one long side of each pastry half.
Place one-sixth of the mixture in a long thin line down the opposite side.
Roll the sausage rolls into a long thin roll. Make sure the joined pastry seam is facing down.
Cut each roll into 4 pieces.
Brush lightly with the egg mixture. Sprinkle with sesame seeds or black poppy seeds (optional).
Place the sausage rolls onto non-stick baking trays.
Cook in the oven for 25-30 minutes or until the pastry is golden and puffed.