Preheat the oven to 230°C (210°C fan). In a medium pot, boil enough salted water for the potatoes. Cut the eggplants lengthwise into 1cm thick slices. Cut the potatoes into 1cm slices as well.
On a parchment-lined baking tray, brush the eggplant slices with 2Tbsp olive oil and sprinkle with a large pinch of salt. Then turn the slices around and also brush the other side with 2Tbsp olive oil and sprinkle with a large pinch of salt. Roast the eggplants in the fully preheated oven for 10–12min. per side until they are golden brown from both sides.
Add the potatoes to the boiling water and cook for ca. 7min. until just cooked. Meanwhile, peel and halve the onion, then cut into thin strips. Peel and finely chop the garlic. Drain and let the potatoes steam in a sieve.
Heat up a medium pan on medium heat with 1Tbsp olive oil. Add the onions and fry for 2–3min. until softened. Then add the garlic and the minced meat and fry for ca. 2min. until the meat starts to brown. Add the passata, 1tsp cinnamon, ½tsp salt, ½tsp sugar and let the sauce simmer for 2–3min. Take off the heat and stir through the potatoes.
Using a fork, whisk 1 egg loose in a medium bowl. Grate the cheese. Mix the béchamel with the egg and cheese and season with a large pinch of salt and a pinch of pepper. Grease an oven dish of ca. 20x20cm. Add ½ the eggplants on the bottom, then add a potato-meat layer followed by the remaining eggplants. Pour over the béchamel mix.
Bake the moussaka in the oven for ca. 30min. until the eggs are completely cooked and the top is golden brown. Meanwhile, make a dressing of ½Tbsp olive oil, ½Tbsp vinegar and a pinch of salt. Just before serving, mix the salad and olives through the dressing. Serve the moussaka with the salad to the side.