Finely chop the carrot, celery and onion. Add the olive oil to a large pan and add the chopped vegetables, sauté the veg gently until soft but not browned (around 7-8 minutes). Finely slice the mushroom and add them to the sauted vegetables.
Fry the mushrooms for 2 minutes then add the pureed tomatoes/passata with the crushed garlic and a pinch of salt and pepper. Stir everything together and add the red wine.
Let the ragu simmer gently for 10 minutes then add the lentils. If the sauce reduces too much add a little water to the sauce before the lentils are added. Stir the lentils into the sauce and simmer gently for another 2 minutes, turn the heat off.
To make the bechamel sauce add the butter to a sauce pan over a low heat, once melted add the flour and stir into a paste. Stir the paste for around 30 seconds then add half of the milk, whisk the milk to get rid of any lumps. Once it starts to thicken add the rest of the milk and whisk until it starts to thicken.
Turn the heat off and add the grated cheese, parmesan, nutmeg and a pinch of salt and pepper. Stir the sauce until the cheese has melted and set aside.
Preheat the oven to 200°C/400F/gas mark 6. Add 1-2 tablespoon of lentil ragu to the bottom of a baking dish and spread it around. Add a layer of fresh lasagna sheets (you may need to cut some sheets to fit your dish). Add another 2-3 tablespoon of ragu followed by 2-3 tablespoon of bechamel sauce (keep ¼ of the bechamel sauce for the top layer). Repeat this until all the ragu has been used up, ideally, you want around 6 layers. Finish with the final ¼ of bechamel sauce.
Cover the lasagne with tin foil and bake in the oven for 20 minutes. Remove from the oven and top with the mozzarella, bake uncovered for 10-15 minutes until the cheese has melted, browned and bubbling.
Let the lasagne rest for 5-10 minutes before serving.