Begin by cooking the lentils. Place the lentils in a sauce pan with water and bring to a boil. Simmer for 10 minutes until the lentils are tender. Drain and set aside. (you can skip this step if you use canned lentils)
Preheat oven to 200°C/400°F.
Chop the onion and mushrooms into big chunks. Heat a drizzle of oil in a large frying pan and add the onion and mushrooms. Fry for a couple of minutes until softened and any liquid has evaporated. Add the soy sauce and fry for another 2 minutes.
Pop the mushrooms and onion in a food processor together with the lentils, tomato paste, parsley, breadcrumbs and a pinch of salt & pepper.
Blend briefly until just combined. It needs to be easy to stick together but still retaining some texture. (if the mixture is too wet, add some more breadcrumbs until you have a slightly stiff mixture)
Unravel the puff pastry and slice in half. Place the mixture on the pastry in a sausage shape. Fold the pastry over and seal with a fork.
Cut into even pieces as big or small as you wish. Brush the tops with a beaten egg and bake for 20-25 minutes until golden brown.