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.
Divide the eggplants over two parchment-lined baking trays. Brush the slices on each tray with 2Tbsp olive oil and sprinkle with a large pinch of salt. Then turn the slices around and repeat the process. Roast the eggplants in the fully preheated oven for 10–12min. per side until they are golden brown from both sides. Swap the trays halfway through baking.
Add the potatoes to the boiling water and cook for ca. 7min. until just cooked. Meanwhile, peel and halve the onions, then cut into thin strips. Peel and finely chop the garlic. Drain and let the potatoes steam in a sieve.
Heat up a large pan on medium heat with 2Tbsp 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, 2tsp cinnamon, 1tsp salt, 1tsp sugar and let the sauce simmer for 2–3min. Take off the heat and stir through the potatoes.
Using a fork, whisk 2 eggs loose in a medium bowl. Grate the cheese. Mix the béchamel with the eggs and cheese and season with ½tsp salt and a large pinch of pepper. Grease an oven dish of ca. 35x25cm. 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 1Tbsp olive oil, 1Tbsp vinegar and a large pinch of salt. Just before serving, mix the salad and olives through the dressing. Serve the moussaka with the salad to the side.