Pour 800ml boiling water over the dried porcini and leave for 10 mins until hydrated. Meanwhile, pour 1½ 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 minute 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 the lid on.
Meanwhile, heat a large frying pan. Add 1½ tbsp oil and tip all of the mushrooms into the pan, including the rehydrated ones. 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 saucepan.
Stir in the Marmite, then 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.
Heat oven to 180C/160C fan/gas 4. Put the potatoes into a pan of cold salted water. Bring to the boil, then cook until mashable.
Meanwhile, mix the remaining 5 tbsp oil with the oregano, then brush the aubergine slices with most of it and sprinkle with sea salt. Griddle for 3 mins on each side until soft.
Drain and mash the potatoes with the soya milk. Season to taste.
Spoon the ragu into a large lasagne dish (or two smaller ovenproof dishes), layer in ½ the aubergine, followed by the mash. Brush the remaining oregano oil across the mash, then finish by topping with the remaining aubergine slices. Bake in the oven for 25-35 mins until golden and bubbling.