Bring a large pan of water to the boil. Put the aubergines in a colander, place a pan lid on top, and place the colander over the water. Steam the aubergines for 25 minutes, turning halfway through.
Pre-heat oven to 180 degrees.
Allow the aubergines to cool slightly, cut in half lengthways and scoop out the flesh. Finely chop the flesh and set aside.
Heat oil over medium heat, add finely chopped garlic, onion and aubergine, add thyme** and chilli flakes. Cook for 10 minutes, stirring occasionally.
Add plum tomatoes and break up with a spoon. Add lentils, balsamic vinegar, juice of half a lemon and chopped walnuts.
Season well, stir and let simmer for 15 minutes.
While the sauce is simmering grate the cheddar and slice mozzarella.
In an ovenproof dish layer sauce, grated cheese and then lasagne sheets. Repeat until you're out of the sauce.
Scatter mozzarella on top, and cook for 25 - 30 minutes until the mozzarella is melted and golden.
Remove from the oven, scatter over a few more chilli flakes and let stand for 10 minutes before serving.