Pour 800ml boiling water over the dried porcini and set aside for 10 mins. Meanwhile, pour 1½ tbsp oil into a large pan and gently cook the onion, carrot, celery and a pinch of salt, stirring for 10 mins, until soft. Remove the porcini and roughly chop. Set aside with the liquid.
Add the garlic and thyme. Cook for 1 min, then stir in the purée and cook for 1 min. Pour in the wine. Cook until reduced, then add the lentils, mushroom stock and tomatoes. Bring to the boil, then reduce and simmer with a lid on.
Meanwhile, heat a large frying pan. Add the 1½ tbsp oil, then tip in all of the mushrooms. Fry until the water has evaporated and the mushrooms are golden. Pour in the soy sauce and stir, then add the mushrooms to the lentil pan.
Stir in the Marmite. Cook over a medium heat for 30-45 mins, stirring occasionally, until the lentils are cooked. Remove the thyme and season.