Preheat an oven to 190C. Get two large non-stick baking trays out and add a good splash of olive oil, you want the bottom of the tray to have a thin coating of oil all over it.
Arrange the aubergine slices on the trays so that you have a single layer on each tray; a few overlapping is fine because they will shrink as they cook. Drizzle a little more oil over the aubergines and then place in the oven for 20 minutes.
Remove the aubergines from the oven and turn them over before roasting again for 15-20 minutes or until golden/dark brown. Remove from the oven and put to one side.
While the aubergines are roasting you can make the other moussaka layers.
For the potatoes
Add the potatoes to a pan of cold, salted water then place on the hob and bring to the boil. Reduce to a simmer and cook for 10-15 minutes or until the potatoes are soft.
Drain the potatoes when they’re cooked and put to one side to cool down.
For the lamb
Get a large frying pan on a medium heat, add a little oil with the onion and fry for 5-10 minutes until the onion is softened. Add the lamb mince and continue cooking until the mince is well browned; break the mince up with a spoon while cooking.
Stir in the spices and garlic and cook for another few minutes. Then add the herbs, tomato paste and tomato with a little salt and pepper and leave to simmer for 10-15 minutes until thickened. Stir from time to time.
While the lamb mixture thickens make the béchamel sauce.
For the béchamel
Add the butter to a saucepan on a low/medium heat and leave to fully melt. Add the flour and stir together well, cook for a minute or two.
Add the bay leaf and then gradually pour in the milk, whisking the whole time, then continue whisking while you bring the sauce to a simmer. Keep whisking to ensure there aren't any lumps in the sauce.
Remove from the heat and add a little salt, plenty of pepper and the nutmeg.
To make the moussaka
Layer the moussaka up in an ovenproof dish putting the lamb at the bottom, then the aubergines and top with the potatoes. Pour the béchamel sauce over everything then bake in the oven at 180C for 20-30 minutes until turning golden brown on top.
I like to serve my moussaka with a big tomato salad but a crisp green salad on the side would be great too.