Remove the stalks from the aubergines, and cut in half widthwise so they’re easier to manage. Cut each piece into thin slices.
Drizzle some olive oil into a large flat-bottomed frying pan, and add a single layer of aubergine slices. Cook the aubergine for a few minutes each side, until slightly golden, and soft and pliable. You’ll probably need to cook the aubergine in several batches, so when the first batch is cooked, transfer the slices to a plate, and repeat with the next batch until you've used all the slices. You may need to add a fresh drizzle of olive oil for each batch.
While the aubergine is cooking, you can prepare the lentil mixture. Heat a dash of oil in a saucepan, and add the chopped onions and garlic. Cook over a medium heat for 5 minutes, until soft and translucent. Add the chopped tomatoes, tomato puree, red lentils, vegetable stock, dried herbs, and a good pinch of black pepper. Mix well to combine, and bring to a simmer. Cook for around 15-20 minutes, stirring regularly, until the lentils are soft (you can add a dash more water if needed).
Wash the potato, and cut it into very thin slices - I used a mandoline. The thin slices allow you to add the potato to the moussaka without pre-cooking, but if you don't have a mandoline, or you just prefer thicker slices of potato, parboil the potato slices for 5 minutes before using.
To begin assembling the vegetarian moussaka, lay half of the aubergine slices in the bottom of a large baking dish (mine measured approx. 12 x 7 inches). Add the lentil mixture on top, then a layer of potato slices, and the remaining aubergine.
To make the béchamel sauce, melt the butter in a small saucepan, and add the flour. Mix well to create a roux (a thick paste), and cook for a minute or two over a medium-low heat. Add the milk a dash at a time, stirring until smooth each time before adding more (find more detailed instructions for how to make a white sauce here). Season with salt and pepper, and a pinch of nutmeg.
Smooth the white sauce over the dish, and top with a few slices of tomato. Bake at 190°C (Gas Mark 5 / 375°F) for around an hour, until the potato slices are completely cooked (you'll need to cover the dish with foil after 20-30 minutes, once the top is browned to your liking). If using parboiled potato slices, you can bake it for less time.
Leave the moussaka to stand for 10 minutes or so after you remove it from the oven (this allows you to cut it into neat slices), then serve warm.