Heat the oven to 200C/180 fan/Gas 4. Remove the stalks from the aubergines, smear them with oil, prick with a fork, then place on a baking sheet and pop into the oven. Cook for around 25 minutes or until the aubergines have collapsed and are soft. Remove from the oven and set aside to cool.
Meanwhile, heat the oil in a pan over a medium heat and add the chopped onions. Cook for around 10 mins or until soft and starting to turn golden. Add the garlic and shredded lamb and cook for 5 mins. Add the wine/stock and cook for another couple of minutes. Finally, add the tomatoes, tomato puree, sugar, herbs, spices and seasonings, and bring it all to a gentle simmer. Cook gently for 20 mins. Set aside.
To make the bechamel, put the milk onto a low heat to warm. In a separate pan, gently heat the butter and when melted, add the flour, and stirring thoroughly to ensure there are no lumps. Gradually add the warmed milk, a little at a time, continuously stirring over the gentle heat. When the sauce is like a thick cream, and no longer tastes of flour, take the pan off the heat and stir in the two egg yolks, salt and some grated nutmeg. Finally add the grated cheese, keeping a bit to sprinkle over the top. (If you are replacing the bechamel with mashed potato, peel 5 large potatoes, place in a pan of cold water and bring to the boil, then mash with butter, salt and milk.)
To assemble the moussaka, butter the base and sides of a square or rectangular baking tin or dish. Line the bottom with half of the meat mixture. Chop the cooled aubergines crosswise into rounds about 6mm thick, and arrange those over the meat to form an aubergine layer. Top that with the remaining meat, and smooth it over with a wooden spoon or spatula. Finally, pour over the bechamel (or mashed potato), smooth it over, then sprinkle over the remaining grated cheese. Cook for 50 minutes, or until the top has started to brown. Make sure not to burn it, so check after 30 or 40 minutes and if it’s colouring too fast, cover with foil. Leave to rest for 10 mins before serving with a crisp salad or slaw.