Prepare the eggplants: Cut the 4 eggplants into slices. Dissolve 1 tablespoon of salt in 1 litre of water, add the eggplant slices, and leave them to soak for 10 minutes.
Rinse the eggplants: Drain the salted water and rinse the eggplant slices thoroughly.
Bake the eggplants: Brush the slices with olive oil and bake at 180°C (356°F) for approximately 15 minutes.
Prepare the potatoes: Peel and slice 4–5 large potatoes.
Fry the potatoes: Heat 300 ml of vegetable oil in a frying pan. Fry the potato slices until golden, then transfer them to a plate lined with paper towels to absorb the excess oil.
Brown the minced beef: Heat 30 ml of extra virgin olive oil in a pot. Add 650 g of minced beef and cook over medium-low heat, breaking it apart until evenly browned.
Add the onion: Finely chop 1 large onion and add it to the meat. Stir well and cook until softened and translucent.
Season the meat: Add salt, freshly ground black pepper, allspice, and 1 cinnamon stick. Stir to release the aromas.
Add the tomato paste: Stir in 2 tablespoons of tomato paste and cook for 1–2 minutes to develop its flavor.
Simmer the meat sauce: Add 400 g of canned tomatoes and 300 ml of water. Stir well and simmer over low heat for 30–40 minutes, until the sauce thickens.
Start assembling the Moussaka: Arrange a layer of fried potatoes in a 20 × 30 cm (8 × 12-inch) baking dish. Cover with a layer of baked eggplants.
Add the meat sauce: Spread the minced beef sauce evenly over the eggplants.
Complete the layers: Arrange another layer of fried potatoes and baked eggplants over the meat sauce.
Prepare the béchamel base: Melt 100 g of butter over medium heat. Gradually add 100 g of flour, stirring continuously, and cook for 2–3 minutes.
Add the cold milk: Gradually pour 1 litre of cold milk into the butter and flour mixture, whisking continuously to prevent lumps.
Add the egg and seasonings: While the mixture is still cool, add the egg and whisk thoroughly until fully incorporated. Season with salt, black pepper, and nutmeg. Continue cooking over medium heat, stirring constantly, until the béchamel becomes smooth, thick, and creamy.
Add the béchamel: Pour the prepared béchamel sauce evenly over the assembled Moussaka.
Add the cheese: Sprinkle grated cheese over the béchamel topping.
Bake the Moussaka: Bake at 180°C (356°F) for approximately 40 minutes, until the top is beautifully golden.
Rest before serving: Let the Moussaka rest for approximately 30 minutes before slicing, allowing the layers to settle and hold their shape.