Clean and wash the aubergines and zucchini and cut them into 1/2 centimetre thick slices. Sprinkle aubergine slices with salt and stand for 15 minutes
Peel and finely chop the onions and garlic. Heat two tablespoons of olive oil. Brown the minced meat in it, add onions and garlic. Drain the tomatoes, dice roughly and steam briefly.
Season to taste with salt and pepper. Wash oregano (put something aside for garnishing), chop roughly and add to the minced meat with breadcrumbs and a whisked egg. Peel, wash and thinly slice the potatoes.
Dab aubergines dry. Fry the prepared vegetables and potatoes separately in the remaining oil until golden brown, drain. For the béchamel sauce, heat the fat. Sauté the flour in it, add milk and bring to the boil while stirring.
Whisk remaining eggs and Parmesan cheese. Mix with some béchamel sauce, then stir into the sauce (do not boil any more!). Season to taste with salt and pepper. Layer the potatoes in flakes in a greased, ovenproof casserole dish (30 centimetres long).
Cover with 1/3 of the sauce. Spread the minced meat on top. Place zucchini and eggplants alternately on top. Spread the rest of the sauce over it. Bake in the preheated oven (electric: 175 °C/ gas: level 2) for 45 minutes.
Serve moussaka sprinkled with remaining oregano.