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Pour 400ml boiling water over the dried porcini and leave for 10 mins until hydrated. Meanwhile pour 1 tbsp oil into a large saucepan. Add the onion, carrot, celery and a pinch of salt. Cook gently, stirring for 10 mins until soft. Remove the porcini from the liquid, keeping the mushroomy stock and roughly chop. Set both aside.
Add the garlic and thyme to the pan. Cook for 1 min then stir in the tomato purée and cook for a min more. Pour in the red wine, if using, cook until nearly reduced, then add the lentils, reserved mushroom stock and tomatoes. Bring to the boil, then reduce the heat and leave to simmer with a lid on.
Meanwhile, heat a large frying pan. Add the remaining oil, then tip in the chestnut, portobello and rehydrated mushrooms. Fry until all the water has evaporated and the mushrooms are deep golden brown. Pour in the soy sauce. Give everything a good mix, then scrape the mushrooms into the lentil mixture.
Stir in the Marmite and continue to cook the ragu, stirring occasionally, over a low-medium heat for 30-45 mins until the lentils are cooked and the sauce is thick and reduced, adding extra water if necessary. Remove the thyme sprigs and season to taste.
Cook the spaghetti in a large pan of salted water for 1 min less than packet instructions. Drain the pasta, reserving a ladleful of pasta water, then toss the spaghetti in the sauce, using a little of the starchy liquid to loosen up the ragu slightly so that the pasta clings to the sauce. Serve topped with fresh basil and some black pepper.